330 Proceedings of the Royal Irish Academy. 



To illustrate this viev, let it be assumed ttat one of our hemi-. 

 spheres, having attained its maximum elevation, is next to undergo 

 subsidence. During this elevated period the land surfaces of moderate 

 height would be in what may be termed Va.Q first stage of depositional 

 action, viz., the formation of subasrial, fresh-water, and estuarine 

 deposits : in the second stage they would be under marine conditions, 

 producing littoral and deepish-water conglomeratic, arenaceous, argil- 

 laceous, and calcareous beds : in the third stage they would be under 

 pelasgic conditions, developing limestones, argillites, and siliceous 

 rocks. [N'ext, elevation having supervened, t\iQ fourth stage would be 

 a repetition of the second, yielding comparatively shallow-water 

 marine deposits, and terminating by passing into the first stage. 

 Thus would our continents, notwithstanding their being at present at 

 an average height of a few thousand feet above the sea-level, become 

 over-laid by vast deposits of all kinds — those of any given stage being 

 the equivalent of one of the formations, usually four, which consti- 

 tute a geological system of rocks (take, for example, the Carboni- 

 ferous System) ; and, moreover, the whole agreeing with the 

 formations of a system in their usual order of superposition. 



A few points must be briefly added. It is not assumed that all 

 great vertical movements have proceeded in the invariable course, and 

 to the extent, vertical, or areal, as above illustrated ; nor that they 

 were unaccompanied by minor ups and downs. The hemisphere oppo- 

 site to the one given as an illustration would be undergoing counter 

 vertical movements. As to the deposits which were thrown down 

 over the abysses of the oceans (Atlantic and Pacific) when the conti- 

 nents were under pelasgic conditions, it is admitted they involve some 

 questions difficult to answer, whether considered in connexion with 

 Dana's hypothesis, or the author's. 



Obviously great recurrent climatal changes would result fi'om 

 these elevations and depressions; severe glacial conditions accompany- 

 ing the one, and the replacement of the latter by genial ameliorations 

 consequent on the other. 



The consideration of these points gives rise to the question, often 

 debated ; — how has it happened that after the Pliocene period climatal 

 conditions prevailed which converted a great portion of Europe and 

 l^orth America (there are grounds for excepting IS^orthem Asia) into 

 ice-covered regions ; and that during the Miocene, or probably some 

 portion of the Pliocene period, areas lying fi'em 10° to 20° of the 

 I^orth Pole, as it appears to some geologists, have enjoyed a climate 

 approaching in genialness that of the south of Europe at the present 

 day. The writer, rejecting all the hypotheses that have been offered 

 in explanation of these climatal interchanges, maintains that they are 

 mainly due to the afore-mentioned vertical movements. 



Confining himself to the climatal conditions which characterised 

 Grinnell Land, Spitzbergen, and other Arctic areas during the Miocene 

 period, as indicated by their plant- remains, he suggests that these 

 and adjacent areas stood at a somewhat lower level relatively to the 

 sea than at present; and formed an archipelago, freely permitting 



