GENERAL REMARKS ON GNEISS. 



important question is, to what extent it partakes of the 

 nature of a mechanical deposit, or whether it may be 

 considered as purely chemical. There are in this case 

 two grounds of judgment, the form of the rock and its 

 internal structure. In the first circumstance it resembles 

 those rocks of later origin of which the mechanical nature 

 is unquestionable. But even these present in many cases, 

 whether they are primary or secondary, the indications 

 of a chemical nature combined with a mechanical struc- 

 ture.' So far therefore the mineral composition of gneiss 

 presents no argument against the belief that it was ori- 

 ginally deposited in a manner similar to that of the other 

 stratified rocks ; a supposition confirmed by the prolonged 

 and parallel direction which it bears to these throughout 

 the whole of Scotland. A stronger confirmation of this is 

 supplied by the appearances of some of the rocks with 

 which it alternates. It will hereafter appear in the history 

 of the red primary sandstone, that some of the beds 

 of that substance bear the same marks of undulation 

 on the surface which characterize the secondary sand- 

 stone and prove its deposition from water, these being 

 found in alternation with the gneiss of that district in 

 which it occurs. It will also be shown in the history 

 of Garvh Island, that a rock containing organic remains 

 is found in a similar position ; both instances equally 

 proving that like those strata it must originally have been 

 deposited from water, whatever changes it may since have 

 undergone. With respect to internal character, it has 

 not hitherto, I believe, presented any decided marks of 

 that mechanical arrangement, which, with the exception 

 perhaps of limestone, are occasionally present in every 

 other stratified rock. I shall hereafter show that the 

 parallelism of mica is not a proof of such arrangement, 

 since that mineral occurs in this manner in veins of 

 porphyry and of trap. The conglomerate gneiss described 

 in Rasay, and also found in Morven, appears to be of 

 posterior formation, and to be analogous to the local 



