202 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 21, 



fragments being principally formed of Longmynd grits and of the 

 Lower Silurian slates and igneous rocks east of the Stiper Stones, we 

 get an approximation to the date of the denudation of those great 

 masses of strata that once entombed the more ancient palaeozoic rocks. 

 In other words, they must, in part at least, have been stripped from 

 the hills that they enveloped before or during the Permian period, 

 otherwise the underlying rocks would not have been reached and 

 degraded by the help of the glaciers. 



Objections answered. — There is a common objection not yet ex- 

 ploded that may be raised to the view taken in this paper. I allude 

 to the argument, that, the earth having gradually cooled, by radia- 

 tion from its circumference, down to a late Secondary, or even Ter- 

 tiary period, this radiation affected the entire climate of the world, 

 and gave tropical characters to its fauna and flora far down in geo- 

 logic time. To treat this subject in detail would lead to a discussion 

 too lengthened and elaborate to be introduced as a subordinate part 

 of a memoir the main object of which is simply to explain the origin 

 of what seems to be an ancient boulder-clay ; but, as the question of 

 an ancient tropical uniformity of climate is still frequently asserted, 

 it cannot in this place be altogether omitted. 



Regarding the palaeozoic faunas, many palaeontologists are of 

 opinion that there is no ground whatever for attributing to them a 

 tropical character. This was certainly the opinion of our late la- 

 mented President, with whom I have often conversed on the subject. 

 Further, the different assemblages of species in equivalent formations 

 in various localities, even in Silurian times, would seem to indicate 

 that the laws of distribution were the same then as now. Neither 

 has it ever appeared to me that the style of reasoning is at all con- 

 clusive which asserts that the Secondary faunas were necessarily 

 tropical because of the peculiarities of form. Of late tertiary date 

 there was an age when elephants ranged every latitude from India to 

 the confines of the Arctic Circle. Is there any reason why at an 

 earlier period Ammonites, Belemnites, and great Saurians should 

 not have done the same ? What applies to animals may apply to 

 many plants ; and, if this be insufficient, we have in the arguments 

 enforced by Sir Charles Lyell respecting different distributions of sea 

 and land good cause for many variations of climate*. 



* The greatest difficulty in the case seems to he in the occurrence of Coal- 

 measure plants in places beyond the Arctic Circle, in Bear Island and the Green- 

 land shores, the loose cellular structure of which plants would seem to indicate 

 that they could neither have been long withdrawn from the stimulus of light, nor 

 yet have endured the long-continued action of frost. (Dr. Hooker, Memoirs of 

 the Geol. Survey, vol. ii. part 2. p. 396.) The whole problem is in many ways 

 obscure, and it is probably a mixed question, some of the elements of which we 

 have not yet got hold of; but it may surely be assumed that the dogma of uni- 

 versal tropical climates dependent on central heat, so far from being proved, is 

 daily losing ground. One argument may be adduced against it, which I think is 

 deserving of attention. 



The average melting-point of ordinary lavas is said to be something intermediate 

 between those of silver and copper, or about 1934°. Assuming the increment of 

 internal heat as we descend to average 1° for every 60 feet below the first 60 feet, 



