Vol. 50.] ANNIVERSARY ADDRESS OP THE PRESIDENT. 93 



that much of tho Torridon Sandstone is of the nature of an arkose , 

 where the felspar has suffered very little from leaching — where, 

 indeed, the rock is chemically unexhausted — it is not surprising 

 that such a rock lends itself to metamorphism of any kind much 

 more than one whose composition is simpler. As to the Archaean 

 rocks, the thicker slices successfully resisted the extremity of 

 metamorphic action. Not, they say, until we reach the point 

 where powerful thrusts follow each other in rapid succession, re- 

 peating thinner slices of the old Archaean platform in the overlying 

 quartzites, is the post-Camhrian shearing strongly marked in the 

 Archaean rocks. At length, in the zone of green schist and sheared 

 gneiss underlying the Moine Thrust, each divisional plane or folia- 

 tion-surface is a shear-plane developed by these post-Cambrian 

 movements. 



For my own part, I am disposed to believe that this wonderful 

 piece of dynamic metamorphism has no great depth in it. We may, 

 however, agree in a general sense with the Authors, in their claim 

 that these facts furnish a large amount of evidence in support of 

 the theory that regional metamorphism is due to the dynamical and 

 chemical effects of mechanical movements acting alike on crystalline 

 and clastic rocks. But, although we may admit likewise that, in a 

 certain sense, regional metamorphism need not be confined to any 

 particular geological period, yet there are degrees in these matters, 

 and surely, when it comes to a question of amount, the balance of 

 result will be largely on the side of the Archaean metamorphic 

 forces. In making this calculation I must confess that I never had 

 much faith in an attempted explanation of the origin of the Moine 

 Schists, whose crystalline character, notwithstanding their inclusion 

 of Cambrian fragments, is probably of much older date than the 

 movements which brought them into their present position. Never- 

 theless the Moine Schists make a very hard nut to crack, and possibly 

 Prof. Bertrand's suggestion, that they are more or less to be likened 

 to the micaceous schists and phyllades which in France form the 

 top of the crystalline series, may not be very far from the mark. 



The Fundamental Bocks. 

 These may roughly be divided into three categories, viz. the sedi- 

 mentary series, the volcanic series, and the crystalline schists. The 

 first includes the unfossiliferous ' Cambrian' of Wales, the Longmynd 

 rocks, the Torridon Sandstone, etc. The volcanic series may perhaps 

 include the Pebidian of Wales, the Uriconian of the West Midlands, 

 etc. ; but as the entire volcanic series of Britain has already been 



