Chapter four covers the subject of measuring time via two particular mechanisms: tree rings (dendrochronology), and radioactivity (radiometric dating). Fossils are also used as an accessory to radiometric dating. Because I'm limiting my criticism to those aspects which can be addressed by an educated layman, criticism of this chapter will be relatively short. There is, no doubt, much more that an expert on these subjects could say, but I will leave such counter-arguments to the experts, dwelling instead on the deficiencies of the argument from the perspective of the intended non-expert audience. There is also a fair bit of background information in this chapter which I need not address.
The opening part of the chapter, from pp.85--88 is background information on the nature of clocks, on which I have no comment.
On pp.88--91, Dawkins introduces tree rings as a dating mechanism. I have several reservations about his argument. First, note that he claims that this mechanism is accurate "literally to the nearest year" (p.88). I expect to see some real-world examples of this accuracy, where an event with a date that is already known
is dated using tree rings, and the tree ring date produces the correct result. Dawkins, however, presents no such examples. Given that there are various factors which can accelerate and retard growth (p.88), isn't it possible that a transient condition such as a local flood could temporarily accelerate growth, producing a ring in a much shorter time frame? Dawkins' assertion about the accuracy of the method seems to be based squarely on the assumption that rings are generated at the precise rate of one per year, but this assumption remains unfounded: no evidence is presented to support it, and the reader has no way to judge how true it is.
There is a similar issue with the question of matching separate wood samples to produce overlaps which allow for dating beyond the lifespan of a single tree (pp.89--90). Intuitively speaking, this seems like a pretty good approach, but I'm not happy about relying on intuition when deeper analysis is possible. Unfortunately, Dawkins does not mention anything about the statistical reliability of the matching process, which should at least be amenable to mathematical analysis. An error in matching could produce a severely inaccurate result, so it's important that we have some measure of statistical confidence in the match.
On p.90, Dawkins says that "dendrochronology in practice takes us back only about 11,500 years." This is, I understand, based on his previous assumptions about rings exactly corresponding to years. I note that the assumption would not need to be off by much -- maybe a factor of two or three -- to fit within a biblical time-scale. Even if we grant the accuracy of the assumptions for the sake of argument, trees dating back 11,500 years can hardly be considered compelling evidence either way in answering whether those trees were a product of creation or evolution: it only tells us that they had already evolved into or been created in that form at that time.
Dawkins mentions in passing some related dating mechanisms (p.90), which I also note in passing are based on similar assumptions of events being strictly annual.
The remainder of the chapter (pp.91--107) deals with radiometric dating (punctuated occasionally by snide remarks about creationists). Pages 91--96 are background information on atoms and radioactive decay, on which I have no particular comment. On p.96, a couple of important points are introduced. One is an explicitly stated assumption: the assumption that none of the daughter product (argon-40 in this case) is present in the initial conditions. The other important point is that only igneous rocks are amenable to dating in this manner.
On the subject of initial conditions, Dawkins says (p.97), "at the moment when molten rock solidifies, there is potassium-40 but no argon." I find this to be a surprising claim. If there is potassium-40 present, then it is decaying into argon-40 at the usual rate, regardless of whether the surrounding rock is solid or molten. I wouldn't expect individual atoms of argon to just bubble out of molten rock -- molten rock is pretty thick stuff, and my experience with bubbles is that their mobility is somewhat proportional to their size. When molten rock containing potassium-40 solidifies, I would therefore expect to find some argon-40 already present. This might be reduced in the case of a lava flow with a lot of exposure to the atmosphere (giving gaseous argon a chance to escape), but not so much when the rock was formed underground.
It could be that my understanding of the process is simply lacking, but it's Dawkins' job to ensure that he presents the appropriate evidence. One way to address this issue would be to take freshly solidified rock (where "freshly" is measured on the potassium-40 scale of billions of years), such as any volcanic rock produced in recorded history. It should be possible to analyse such rock and show that its argon-40 content is nearly zero (relative to its potassium-40 content). If it is, it raises the question as to where all the argon-40 went, but that's a different matter. It might also be interesting to see if there is any difference based on the type of rock: obsidian and pumice are both volcanic rocks, but the former is glassy, whereas the latter is full of bubbles. Unfortunately, Dawkins cites no such controlled tests, so we have no idea how good the assumption is.
On pp.97--101, Dawkins describes the relationship between igneous rocks and the geologic column, which is a model of the earth's sedimentary rocks based on fossils and other factors. He starts with the observation that "fossils are almost never found in igneous rock," but rather in sedimentary rocks. He mentions in passing that sedimentary rock is "gradually laid down on the floor of a sea or lake or estuary", and "the sand or mud becomes compacted over the ages and hardens as rock". Similarly, "corpses that are trapped in the mud have a chance of fossilizing." This description of rock formation and fossilisation is presented as given, which I find problematic, because it already assumes long ages and gradual processes -- which is exactly the sort of thing that this chapter is trying to prove. It's poor logical form to let a thing you are trying to prove become an assumption you make (it's called circular reasoning), so Dawkins really ought to present evidence to back it up. It's not obvious to me how one might go about that in this case: after all, if something occurs "over the ages," how does one experiment with it in a relatively fleeting human time frame? Still, the argument hasn't quite become circular yet: we may proceed with caution, bearing in mind the long-age assumptions that have been introduced.
On pp.97--98, Dawkins notes that sedimentary rocks can't be directly dated, due to their mixed composition. Instead, he suggests that the best we can do "is to use the dates of igneous rocks found near sedimentary rock, or embedded in it." There are two points to note about this. First, the problem that he cites with dating sedimentary rock has a partial loophole: we can safely assume that all the particles in a sedimentary rock were solid when the rock formed, so the date of any given particle will be older than the date of the sedimentary rock as a whole. This means that any date measurement of a sedimentary rock provides an upper limit
on the age, rather than an age estimate. That's not as useful as an actual age, but it still has its uses, particularly as a cross-checking mechanism. I haven't heard of this technique being used in practice, however. Maybe it won't work -- but if that's the case, then the dating process isn't quite as simple as Dawkins has made it out to be.
The other point to note is that the argument is progressing based on the gradualist, long-age assumptions I noted earlier, bringing it closer to circularity. In order to date the sedimentary rocks on the basis of nearby igneous rocks, we must rely on gradualist assumptions: the idea that the earth has (as a general rule) been accumulating its rock layers slowly and gradually, so that things physically near each other in the rocks are close to each other in the temporal sense as well -- minus various exceptions where they aren't, for whatever reason. Dawkins' assertions about the age of sedimentary rocks is thus based at least in part on assumptions of gradualism, which in turn implies long ages, so by the time you've decided that the age of igneous and sedimentary rocks are related, you've already decided that the ones near the bottom are millions of years older than the ones near the top. To be fair, this isn't a hard and fast rule: there are possible arrangements of rock where one or the other clearly came first, and the igneous rock could therefore act as an upper or lower limit on age (if we grant that it can be dated accurately), but Dawkins is not being this conservative in his application of dating.
In discussing the geologic column, Dawkins notes the similarity of various layers around the world (p.98): "they are recognisably similar to each other, and they contain similar lists of fossils." When this is interpreted in the light of gradualism and long ages, it implies that each layer is a long period of time, and that each layer contains fossils representative of the globe during that era. Thus, Dawkins says (p.99), "long before we knew how old fossils were, we knew the order
in which they were laid down, or at least the order in which the named sediments were laid down." Where one layer is physically on top of another layer, this isn't a controversial statement -- although even in those cases there may be cause to plead that the layers have been subsequently altered (p.99) -- but when rocks in one area are deemed to be older than rocks in another area due to indirect layering relationships, gradualism is a key assumption.
On p.99, Dawkins makes a pre-emptive strike against an accusation of circular reasoning: strata are identified by the fossils they contain, and the ordering of the fossils is evidence for evolution, but he says that this is not a circular argument. I agree that this is not circular, but both aspects rely on a prior assumption of gradualism and long ages. It is the relationship between gradualism and age determination which presents the danger of circularity: gradualism implies long ages, so if you reach a conclusion of long ages based on gradualist assumptions, you've merely concluded what you already assumed.
On pp.99--100, Dawkins asserts that the fossil record provides evidence for evolution on the basis that "certain kinds of fossils, for example mammals, appear only after
a given date." I think that calling this "powerful evidence for evolution" is a little over the top. Aside from any other objection, it's a very superficial analysis of the data: there's a lot more to evolution than animals not appearing in the fossils before a certain date. Sure, it's one of the things that you would expect if long-age evolution were the correct explanation for the fossil record, but it's only one isolated fact. There may be a little cherry-picking going on here in the presentation of the evidence (as we will see shortly).
Another point that Dawkins makes on p.100 is that the fossil record could have been other than it is, and this leaves evolution vulnerable to falsification. "At any moment somebody might dig up a mammal in Cambrian rocks, and the theory of evolution would be instantly blown apart if they did." This is, I think, less impressive than he makes it sound: the general structure of the fossil record was known to Darwin, and he shaped his theories around it. One hardly needs to be an evolutionist to feel safe in predicting that we will never find a mammal in Cambrian rocks: we have a long history of not finding them there, regardless of any theoretical basis for the fact. Anyhow, I seriously doubt that most evolutionists -- Dawkins included -- would abandon the theory just because of one anomalous fossil.
On pp.100--101, Dawkins takes a little time out to ridicule creationism. I'm really not interested in hearing about creationism from Dawkins, because I can feel quite sure he won't present it in a flattering light. Even so, turn-about is fair play, so I'll take the opportunity to apply some of the criticism he levels at creationism against his own argument to see if it fares any better. He analyses a "head for the hills" flood theory of the fossil record and finds it wanting because he would expect to see "a statistical tailing off of mammals as you move down through the rocks," as opposed to the sudden introduction that we actually see.
If evolution (and geological gradualism) were true, we would not expect to see particularly distinct species in the fossils at all: we would expect a kind of biological continuum from bottom to top. Darwin understood this, and recognised that this is not what we find in the fossil record. He suggested that the anomaly was a product of the imperfection of the fossil record, or our incomplete study of it. These days the disconnect between the expectation and the facts has been around for so long that nobody seems to care about it any more, so perhaps I should raise a similar but distinct issue.
Less controversially, we expect to see species that exist on earth present in the fossil record during the time that they exist. We don't see mammals in Cambrian rocks, so Dawkins takes it to mean that they didn't exist then. Similarly, dinosaurs only exist in a particular range of rocks, and Dawkins takes it to mean that they evolved and then went extinct. What, then, of something like the fish called the coelacanth, which vanished from the fossil record 65 million years ago (by current evolutionary measures), but is still alive and well today? The coelacanth is not an isolated incident, either: that kind of anomaly is common enough to have its own term -- a "Lazarus taxon"
. Dawkins is willing to dismiss "head for the hills" flood theory on the basis of missing mammals in the Cambrian period. If that's the kind of standard we're applying, then we should reject the theory that the fossils show the history of life on earth because of "Lazarus taxa", such as missing coelacanths.
I think it's obvious that Dawkins applies different standards to different theories. We don't have to care, so long as we focus on criticising his pro-evolution argument, and don't take his anti-creation rhetoric too seriously.
On pp.103--106, Dawkins addresses the subject of carbon-14 dating. Carbon-14 is different to other radioactive isotopes used for dating in that it has a relatively short half-life, and is constantly produced from nitrogen in the upper atmosphere. There isn't much new to add here. Carbon dating has its own set of assumptions, and although Dawkins defends them as reasonable and assures us that everything has been verified as accurate, there is no evidence along the lines of taking an arbitrary bit of wood off a tree and demonstrating that it has a zero date, or such like: we just have to take Dawkins' word for it.
In closing the chapter (pp.106--107), Dawkins takes a moment to reiterate and emphasise that all the dating methods are in agreement: that "different clocks agree with each other -- within the expected margins of error." He applies the terms "confidence" and "agreement" liberally to the methods. The emphasis is necessary, he says, because of the "history-deniers" who constitute such an upsetting portion of the population. All this assertion of reliability is no substitute for actual evidence of reliability, however. The chapter has been primarily about describing
the dating mechanisms rather than giving evidence
of their reliability. Such background information may be a necessary part of the presentation, but the purpose it serves is to allow the audience to understand the significance of the evidence. The mechanism itself is not the evidence.
Dawkins also takes a particular swipe at history-deniers who suggest that rates of radioactive decay may have been different in the past. He calls this a glaring case of special pleading, "and it glares even more when you have to make mutually adjusted special pleading claims for each one of the clocks separately." I'd have thought it rather obvious that no such individual treatment is necessary. If the rate of radioactive decay were to be ramped up universally for whatever reason -- doubled, say -- then the clocks would still continue to agree with each other: they'd just be running twice as fast. Dawkins, however, wants us to "think of the amount of contrived and complicated fiddling with the laws of physics that would be needed in order to make all the clocks agree with each other."
I'd be more willing to take Dawkins' word for the reliability of his supporting methods if it weren't for this obvious and severe bias in his evaluation of things. This chapter has thus served largely to galvanise my scepticism and demand for independent evidence -- evidence which is sorely lacking in this chapter.