Robert Gentry writes the following on his web site at this link:http://www.halos.com/finger.htm "Now in granite there are four different types of Po halos; on occasion two or three types can be seen microscopically in the same specimen of mica. This situation is virtually impossible to reconcile with the hypothesis that such halos formed from U-decay products because the different Po-isotope half-lives mean that greatly different quantities of each isotope will coexist. In particular, since the expected amounts are directly proportional to the different half-lives, this means that at any given time the atomic ratio 210Po:218Po should be about 67,000:1. Thus, if Po halos in biotites were from secondarily-derived Po from U decay, there should exist about 67,000 210Po halos for each 218Po halo. This is definitely not the case. In some mica specimens the number of 218Po or 214Po halos far outnumbers the 210Po halos." There is a major blunder in the logic of the above paragraph. Gentry assumes in the above that the various members of the Uranium decay chain are in radioactive equilibrium. This means that, for example, the activity of Po-210 and Po-218 will be equal. Also, we know that activity equals the number of atoms of a nuclide times the decay constant. So it is possible to calculate, as Gentry has done, the number of atoms of both Po-210 and Po-218. So far, he is correct. But then he states that the number of halos produced by each nuclide should be in the same ratio, that is in the ratio of 67,000 Po-210 halos for each Po-218 halo. This is not right and in fact it is off by a factor of 67,000. Consider the fact that each U-238 atom is the parent of a decay chain that includes both Po-218 and Po-210. One and only one of each of these Polonium atoms is created ultimately by the decay of one parent atom. So if we have 10 lbs of the Uranium parent and let it decay completely away to stable lead, then the potential number of halos will be equal for both Po-210 and Po-218. In other words the instantaneous abundance of Po-210 and Po-218 atoms, for uranium in radioactive equilibrium, does not determine the potential number of halos. This is a mistake in logic that even an expert in radioactivity could possibly make. But such a mistake would probably not get past an informed reviewer. This shows the value of peer review and shows that information found on the web, that is not subject to good review, should be taken with a grain of salt. I suspect that another reason for the mistake is that Gentry does not have an extensive background in dealing with radioactivity, even though he does have experience with halos. Gentry needs to have his polonium halos from primordial polonium in order to support his argument of a recent creation. But the fact of his blunder above weakens his argument to some extent and does not give confidence in his ability to handle this issue.