30 The Scottish Naturalist. 



by which it has been moved into its place. Masses of the Arch- 

 sean gneiss have thus been thrust up through the younger rocks, 

 and pushed far over their edges. When a geologist finds vertical 

 beds of gneiss overlying gently inclined sheets of fossiliferous 

 quartzite, shale, and limestone, he may be excused if he begins to 

 wonder whether he himself is not really standing on his head. 



The general trend of all these foldings and ruptures is from 

 north-north-east to south-south-west, and the steep westward fronts 

 of the folds show that the terrestrial movement came from east- 

 south-east. Corroborative evidence that this was the direction of 

 the movement is furnished by a series of remarkable internal rear- 

 rangements that have been superinduced upon the rocks. Through- 

 out the whole region, in almost every mass of rock, altogether ir- 

 respective of its lithological characters and its structure, striated 

 planes may be noticed which are approximately parallel with the 

 thrust-planes, and are covered with a fine parallel lineation, run- 

 ning in a west-north-west and east-south-east direction. These 

 surfaces have evidently been produced by shearing. Again, many 

 of the rocks near the thrust-planes, and for a long way above them 

 are marked by a peculiar streaked structure, which reminds one of 

 the fluxion-lines of an eruptive rock. The coarse pegmatites in 

 the gneiss, for example, as they come within the influence of the 

 shearing, have had their flesh-coloured felspar and milky quartz 

 crushed and drawn out into fine parallel laminae till they assume 

 the aspect of a rhyolite in which fluxion-structure has been excep- 

 tionally well developed. The gneiss itself coming into the same 

 powerful mill, has acquired a new schistosity parallel with the 

 shearing-planes. Hornblende rock has been converted into horn- 

 blende schist. Moreover, new minerals have, like-wise made their 

 appearance along the new divisional planes, and in many cases 

 their longer axes are ranged in the same dominant direction from 

 east-south-east to west-north-west. 



Murchison believed that the Silurian quartzites and limestones 

 of Eriboll pass up under, and are quite conformably overlain by 

 this upper gneiss. It is quite true that they are so overlain ; but 

 the overlying rocks, instead of having been regularly deposited on 

 them, have been pushed over them. What then, are these overly- 

 ing rocks ? Though they have undergone such intense alteration 

 during the process by which they were moved into their present 

 position that their original characters have been in great measure 



