Dr. Gentry's years of excellent experimental work and observations on radiohalos make him without doubt the world's leading authority on them. However, I have a problem with his view that the "orphan" Polonium halos (the ones unaccompanied by halos from parent nuclides) must be primordial. Why (as Dr. Robert Brown has suggested) are the only orphan halos from Polonium isotopes in the Uranium decay series? Shouldn't there be some halos or daughter products from the other Polonium isotopes as well? It seems to me that there are other possible creationist explanations for the orphan halos. One which John Baumgardner, myself, and others have discussed has the following features:

1. Uranium decays at an early stage of earth history (for example, after the Fall), producing Polonium 210, 214, and 218.

2. Decay stops for a period (say from the Fall to the Flood), during which time the Polonium is physically or chemically separated from the Uranium.

3. Decay restarts (say during the Flood), producing halos in already-existing granite crystals.

This model is new and not well thought out yet. I cite it merely as a contrasting illustration. If someone rises to Dr. Gentry's famous challenge and synthesizes granite, it might prove that the halos are not primordial. But it would not prove that the halos were formed by natural processes working at present rates.

D. Russell Humphreys, Ph.D.
Albuquerque, New Mexico

My essential criticisms of Dr. Gentry's halo interpretations have been published in more detail elsewhere (Physics Today, April 1983, 11-13). The main problems with his thesis are:

1. The inclusion minerals at the centers of halos are nearly always minerals that are known U or Th-bearing minerals like zircon or monazite. These minerals are not geochemically compatible with Group VI elements like Po and there is no reason to believe they would have Po except from decay of U or Th.

2. The only isotopes of Po that Dr. Gentry reports finding are those that form by alpha decay of U and Th. There are 26 isotopes of Po, and the 22 that are not alpha decay products of U and Th have not been reported. These two points strongly indicate that the Po Dr. Gentry finds is due to conventional U and Th decay and is not primordial, unresolved problems notwithstanding.

3. Dr. Gentry alternates between uniformitarianism and non-uniformitarianism as it suits his hypothesis. He accuses orthodox geologists of circular reasoning for assuming that the halos imply constant nuclear decay rates without direct proof, but he assumes (without direct proof) that his halos are due to alpha radiation in the past and (again without direct proof) that he can identify the halos with specific elements. I believe Dr. Gentry is correct when he identifies his halos, but he is correct only because uniformitarianism is valid. Finally, he gratuitously assumes that, if decay rates change, they must slow down with time; couldn't they just as easily be speeding [p. 313] up so that rocks are older than radiometric ages indicate?

Assuming uniformity of physical laws is neither arbitrary nor circular: We live in a universe of patterns, and once a pattern is known to exist, the burden of proof is on someone who asserts that the pattern can change. When our checkbooks fail to balance, we do not assume lightly that someone has tampered with our account; we look for errors in our accounting instead. Similarly, we assume that unresolved problems in science will turn out to have a conventional explanation and only when the evidence becomes incontrovertible do we postulate changes in the laws of nature. As points (1) and (2) above indicate, Dr. Gentry's halos do not come anywhere close to this level of urgency. There is every reason to believe the halos have a conventional origin. In addition, there is no observational evidence that decay rates can change as drastically as they must to accommodate the creationist time scale; there is no theoretical basis for believing that they can change (Barry Setterfield makes a game try, but his treatment is full of errors). The paltry few percent change in electron-capture decay rates that creationists cite fall far short, in degree and in kind, of the million or so times that all forms of decay would have to speed up to reconcile creationist chronology and the radiometric time scale. Until creationists can demonstrate such enormous accelerations of decay beyond any doubt, and that probably means in the laboratory, most geologists will continue to be unrepentant uniformitarians.

Steven Dutch
Green Bay, Wisconsin