Mr. Hicks on the Upper Silurian and Palaeozoic Rocks. 169 



2. " The Physical Conditions under which the Upper Silurian 

 and succeeding Palaeozoic Rocks were probably deposited over the 

 Northern Hemisphere." By Henry Hicks, Esq., E.GLS. 



In this paper the author, after pointing out the lines of de- 

 pression explained in his former paper to the Society, now further 

 elaborated the views then propounded by him by carrying hia 

 examination into the higher Palaeozoic series and into more exten- 

 sive areas. Beginning at the top of the Lower Silurian, where he 

 first recognizes any evidence of a break in the Palaeozoic rocks, he 

 proceeded to show that this break was restricted to very limited 

 areas, and almost entirely confined to the parts which had been first 

 submerged, and where the greatest thickness of sediment had accu- 

 mulated on both sides of the Atlantic, and hence where the prae- 

 Cambrian crust had become thinnest. On the European side this 

 break occurred where volcanic action had taken place, and has 

 doubtless to be attributed to the combined action of upheaval of 

 portions of the crust and the heaping up of volcanic material, the 

 latter in some cases forming volcanic islets of considerable extent. 

 He strongly objected to look upon these breaks, even in the British 

 area, where they are most marked, as evidence of a want of con- 

 tinuity over other and far greater areas, or to admit that, even where 

 there was conformity in the rocks at this point, " great in- 

 tervals of time are indicated unrepresented by stratified formations." 

 The conformity found in extensive and widely separated areas is 

 proof also that a gradual contraction took place of an enormous 

 portion of the crust in the northern hemisphere in Palaeozoic times ; 

 and the breaks at the close of the Lower Silurian and in the De- 

 vonian are not indications of an arrest in the general subsidence. 

 After indicating the changes which must have taken place in the 

 climate from this gradual spreading of the water, and the evidence to 

 be derived from the consideration of the deposits and the faunas, 

 the author drew the following general conclusions : — 



1. That the condition of the northern hemisphere at the beginning 

 of Palaeozoic time was that of immense continents in the higher 

 latitudes, traversed by mountainous ranges of great height, but with 

 a general inclination of the surface, on the one side (European) to 

 the south-west and south, and on the other side (American) to the 

 south-east and south. 



2. That these continents were probably eovered, at least in their 

 higher parts, with ice and snow ; and that much loose material had 

 consequently accumulated over the plains and deeper parts, ready to 

 be denuded off as each part became submerged. This would account 

 for the enormous thickness of conglomerates, with boulders, grits, 

 and sandstones, found in the early Cambrian rocks, and also to a 

 pertain extent for their barrenness in organic remains, 



3. That the depression over the European and American areas 

 as general from at least the latitude of 30 Q northwards ; that the 



arts bordering the Atlantic were the first to become submerged ; 

 he lower latitudes also before the higher. 



4. That the depression could not have been less altogether, for 

 he whole of the Palaeozoic, than 50,000 feet ; and that conform- 



Phil, Mag. S, 5. Vol 1. No. 2. Feb. 1876. N 



