GENERAL REMARKS ON GNEISS. 



to other rocks, whether in the order of precedence, suc- 

 cession, or alternation. It seems to occur in immediate 

 subjacence to all the primary strata, and, in Scotland at 

 least, to the secondary also. The instances in which it 

 is succeeded by micaceous schist and quartz rock are 

 so common as not to need enumeration. In Lewis and 

 North Uist, it immediately precedes argillaceous schist, as 

 it also does in several other parts of Scotland. In Perth- 

 shire and in Sutherland it is followed by primary lime- 

 stone, and by the primary red sandstone ; and, in Morven, 

 it is covered by an extensive tract of that conchiferous 

 limestone accompanied by white sandstone and coal, 

 which occurs in a dispersed manner throughout this 

 coast. 



Occasionally it reposes on granite, but not exclusively ; 

 since the granite of Scotland is followed, in some situation 

 or other, by every one of the primary strata : and that 

 it does not necessarily succeed any one rock, will be 

 proved by the history of its alternations. 



With micaceous schist and quartz rock these are so 

 frequent that examples need not be pointed out. In Isla 

 the gneiss is imbedded in masses of argillaceous schist, 

 or occurs in frequent repetitions with that rock ; while 

 in lona, the schist which is interposed between the granite 

 and the gneiss, is also imbedded in the latter, as it fre- 

 quently, moreover is found, at Loch Eribol, Loch Carron, 

 and in other places, in a much more extensive and un- 

 questionable manner. In Perthshire the gneiss and the 

 limestone alternate on so large a scale that the latter 

 cannot be considered as a subordinate rock. The gneiss 

 of Sutherland and Ross-shire also alternate with limestone, 

 and in the same districts there is a perpetual interchange 

 of its beds with those of the primary red sandstone and of 

 quartz rock. In Sky it has been shown to graduate into 

 these latter rocks, as well as into argillaceous and chlorite 

 schists. 



With respect to its history as a rock species, the most 



