SUPPLEMENTARY NOTE A, 115 



areas would be subjected to marine actions, producing littoral and 

 deepish-water conglomeratic, arenaceous, argillaceous, and cal- 

 careous beds ; in the third stage, in which maximum depth had 

 been reached, they would be under pelagic conditions, developing 

 limestones, argillytes, and siliceous rocks. Next, elevation 

 having again come on, the fourth stage would be a repetition of 

 the second, yielding comparatively shallow-water marine de- 

 posits ; and this would terminate by passing into the fifth stage, 

 which corresponds to the first one. Thus would our continents, 

 notwithstanding their being at present at an average height of 

 a few thousand feet above the sea-level, become overlaid in every 

 systemal period with vast deposits of all kinds those of any 

 given stage representing one of the formations (assumably five) 

 which constitute a geological rock-system, and, moreover, the 

 whole agreeing with the formations of a system in their suc- 

 cessive order of superposition. 



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

 all such vertical movements have proceeded in the invariable 

 course and to the extent, vertical or areal, above illustrated, 

 or that they were unaccompanied by minor ups and downs. 

 The region opposite to the one given as an illustration would be 

 undergoing a cycle of counter vertical movements. As to the 

 deposits which were thrown down over the abysses of the oceans 

 (Atlantic and Pacific) when the continents were under pelagic 

 conditions, it is admitted they involve some questions difficult 

 to answer, whether considered in connexion with Dana's hypo- 

 thesis, or the one just stated. 



Obviously great recurrent climatal changes would result from 

 these elevations and depressions severe glacial conditions 

 accompanying the one, and the replacement of the latter by 

 genial ameliorations arising from 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 North America (there are grounds for 

 excepting Northern Asia, into ice-covered regions, and that 

 during the Miocene (or probably some portion of the Pliocene) 

 period areas lying within from 10 to 20 of the North Pole have 

 enjoyed, as it appears to some geologists, a climate approaching 

 in genialness that of the south of Europe at the present day ? 



