276 ANNUAL OF SCIENTIFIC DISCOVERY. 



crater during an eruption, or from a lava stream during its solidifica- 

 tion. It is more than 40 years sinee Mr. Srrope, in his work on 

 Volcanoes, insisted on the important part which water plays in an 

 eruption, when intimately mixed up with the component materials 

 of lava, aiding, as he supposed, in giving mobility to the more solid 

 materials of the fluid mass. But when advocating this igneo-aqueous 

 theory, he never dreamt of impugning the Huttonian doctrine as to 

 the intensity of heat which the production of the unstratified rocks, 

 those of the plutonic class especially implies. 



The exact nature of the chemical changes which hydrothermal ac- 

 tion may effect in the earth's interior will long remain obscure to us, 

 because* the regions where they take place are inaccessible to man; 

 but the manner in which volcanoes have shifted their position through- 

 out a vast series of geological epochs becoming extinct in one region 

 and breaking out in another may, perhaps, explain the increase of 

 heat as we descend towards the interior, without the necessity of our 

 appealing to an original central heat or the igneous fluidity of the 

 earth's nucleus. 



Recent Geological Changes in the Configuration of Great Britain. 

 It is already more than a quarter of a century since Sir Roderick Mur- 

 chison tirst spoke of the Malvern Straits, meaning thereby a channel of 

 the sea which once separated Wales from Engl.md. That such marine 

 straits really extended, at a modern period, between what are now the 

 estuaries of 'the Severn and the Dee has been lately confirmed in a satis- 

 factory manner, by the discovery of marine shells of recent species in drift 

 covering ihe watershed which divides those estuaries. At the time when 

 these shells were living, the Cot>wold Hills, at the foot of which Bath city 

 is built, formed one of the numerous islands of an archipelago into which 

 England, Ireland, and Scotland were then divided. The amount of ver- 

 tical movement which would be necessary to restore such a state of the 

 surface as prevailed when the position of land and sea were so different 

 would be very great. 



Nowhere in the world, according to our present information, is the ev- 

 idence of upheaval, as manifested by upraised marine shells, so striking as 

 in Wales. In that country Mr. Trimmer first pointed out, in 1831, the 

 occurrence of fossil shells in stratified drift, at the top of a hill called Moel 

 Tryfaen, near the Menai Stniits, and not far from the base of Snowdon. 

 Mr. D.irbishire has obtained from the same drift no less than 5-t fossil 

 species, all of them now living either in high northern or British seas, 

 and 11 of them being exclusively arctic. The whole Fauna be;irs tes- 

 timony to a climate colder than th it now experienced in these latitudes, 

 though not to such extreme cold as that implied by the Fauna of some 

 of the glacial drift of Scotland. The shells alluded to were procured at 

 the extraordinary hight of 1,300 feet above the sea-level, and they demon- 

 strate an upheaval of the bed of the sea to that amount in the time of 

 the living Testucea. A considerable part of what is called the glacial epoch 

 had already elapsed before the shelly strata in question were deposited on 

 Moel Tryfaen, as we may infer from the polished and striated surfaces of 

 rocks on which the drift rests, and the occurrence of erratic blocks 

 smoothed and scratched, at the bottom of the same drift. 



Cause of the Glacial Epoch. The evidence of a period of great 

 cold in England and North America, in the times referred to, is now 



