16 
quently, Sir Wa. THompson declined to assign any limit whatever to the 
possible thickness of an ice sheet. But have geological observers been 
sufficiently careful to give the physicist correct data? Have they shown 
that the ice which covered hill and dale had a level surface, or that, as the 
breadth-of the valleys increased, its surface conformed more and more to the 
configuration of the ground over which it passed, so as to reduce the actual 
thickness in the deeper and larger valleys? Of how much, as a maximum, 
have we direct evidence, either at present or any pasttime? What evidence 
have we that a period of extreme cold in any one area—say N. W. Europe— 
was also a period of extreme cold over the whole Northern Hemisphere, or 
even over the whole of the Eastern as well as the Western part of the Old 
World? Another example of questions for which we must refer to the 
physicist : an account is brought* from Prussia of a deep boring at Speren- 
berg, near Berlin, which should interest us in Cheshire. It is said to be 
through 4,000 feet of pure rock salt, and throughout this depth it is stated 
that careful observations on the rate of increase of temperature have been 
made, the result of which, tabulated by Professor Monr, lead him to the 
following conclusion :—* That the cause of the increasing heat in the interior 
of the earth must lie in the upper strata of the earth’s crust."’ The only 
comment I will offer on the whole is—'* Wonderful, if true.” But still we 
must see that in such questions, whether we are enquiring into the causes 
of metamorphism of volcanic phenomena, or earth movements on a large or 
small scale, our appeal must be to the physicists. What sources of error 
are there in observations on the increase of temperature as we descend ? 
What would be the effect of laying on or removing five or six miles of 
sediment? What influence might the process of crystallization or cleavage- 
producing pressure have? How ought the crust of the earth to crumple if 
a hardened outside was adjusting itself to fit a shrinking nucleus? But 
I feel that geologists have much to do in collecting accurate data. We 
know that there have been successive movements of elevation and depression 
in the same area. Have we clearly shown in what direction the movements 
have proceeded, by carefully tracing advancing or receding shore lines, by 
noticing the source of the sediment, and in many other ways? We call our 
science of geology very new; so we may say of most of the cognate 
branches which are commonly understood to be included under the head 
“Natural Science.” ‘True, the herbalist and medicine-man, in choosing 
healing plants, learnt to distinguish and classify them. The alchemist, in 
his experimental search for something that would turn the baser metals into 
gold, found out many of the facts of chemistry. What would he bave 
thought had he been told that men would, by simply catching some of 
the light from his crucible on a bit of glass, be able to detect quantities too 
small to be found out by the chemist’s skill? ‘The collector of precious 
stones grouped his gems by various external ebvious properties. I have an 
old work, published in Cambridge about two centuries ago, in which one 
Tuomas Nicuots, sometime of Jesus College in Cambridge, divided all 
precious stones into such as are great and such as are small; these groups 
he further subdivided into such as are hard and such as are soft. The 
ancient Hindoos and Egyptians thought that the earth had been many times 
© Nature. vol. xii., p. 545, and vol. xiii., p. 178. 
