The Scott is J i Naturalist. 33 



great series of terrestrial movements. These must have occurred 

 some time between an early part of the Silurian period and 

 that portion of the Old Red Sandstone period represented by the 

 breccias and conglomerates of the Highlands. In the Central and 

 Eastern Highlands, the slates, phyllites, grits, quartzites, and lime- 

 stones, which, along the southern border, are scarcely more altered 

 than their probable equivalents among the Silurian Uplands, had 

 been greatly plicated, and had assumed a more or less crystalline 

 structure. But when these changes were brought about, there lay 

 to the north-west a solid ridge of Archaean gneiss and Cambrian 

 sandstone which offered strong resistance to the plication. The 

 thrust from the eastward against this ridge must have been of the 

 most gigantic kind, for huge slices, hundreds of feet in thickness, 

 were shorn off from the quartzites, limestones, red sandstone, and 

 gneiss, and were pushed for miles to the westward. During this 

 process, all the rocks driven forward by it had their original struc- 

 ture more or less completely effaced. New planes, generally par- 

 allel with the surfaces of movement, were developed in them, and 

 along these new planes a re-arrangement and re-crystallisation of 

 mineral constituents took place, resulting in the production of 

 crystalline schists. This metamorphism certainly occurred after 

 early Silurian times, for Cambrian and Lower Silurian strata, as 

 well as Archaean rocks, have been involved in it. 



It is obvious that into the problems of Highland geology, 

 always admittedly obscure, a fresh element of difficulty is intro- 

 duced. At the same time the aid furnished by a minute study of 

 the southern sections is so great that we may hope to attack these 

 problems with more success than has hitherto seemed probable. 

 The work, too, is not of a kind to be attempted in a few hasty 

 scampers over the ground. It will require patient detailed map- 

 ping. But when the great base-lines have once been accurately 

 traced, the difficulties will doubtless begin to diminish, and, like 

 the pieces of a puzzle, the various segments of the Highlands will 

 then be found to arrange themselves in their proper places. 



ARCH. GEIKIE. 



