NICOL — SOUTHERN GRAMPIANS. 209 



without its interposition. The diversity in mineral character is of 

 less importance, though still very considerable. It may in some 

 measure have arisen from the far less extent of metamorphic action 

 to which the sandstones (quartzites) in the north have been exposed, 

 many portions of them being not more altered or more crystalline 

 than the ordinary sandstones of the Coal-formation. To this more 

 powerful metamorphic action we may also ascribe the more marked di- 

 versity of, and the entire absence of organic remains in, the southern 

 limestones and quartzites, — a want which prevents us from feeling 

 perfectly assured of the identity of these two formations. Whatever 

 view we may take of this question, it is, however, certain that the 

 quartzites and slates of the south-west cannot be of more recent 

 date than the lower portion of the Silurian. As the gneiss and 

 mica-slate are inferior and older formations, we are thus compelled 

 to carry them back into a still more ancient period in the history of 

 the earth. There is indeed nothing in the mere thickness of these 

 deposits to prevent us regarding them, as I was formerly inclined to 

 do, as the altered representatives of the lower portion of the Silurians 

 of the south, as the clay- slates were of the upper portion, — but the 

 break in the conformity of the formations, seen in many places, is 

 opposed to this view. The analogy of nature too in other regions, 

 where the lowest Primordial zone of life rests unconformably on 

 upturned, and evidently far more ancient, crystalline or metamorphic 

 strata, strengthens this view of the great antiquity of the gneiss and 

 mica-slate of the Scottish Highlands. This is especially true of 

 Scandinavia, in which we find a great gneiss-formation identical in 

 many respects with that of Scotland, and probably at one time even 

 in geographical connexion with it. The conviction has thus been 

 forced on me, that in the north of Scotland there is a much longer 

 series of formations — deposits dating from a far more early period 

 in the history of the earth — than might at first be imagined. Thus, 

 as our knowledge of the geological structure of Scotland becomes 

 more precise, we find that, as in every other field of geological science, 

 the duration of the earth, as measured by successive phenomena, 

 irresistibly expands, and we are more and more taught to feel how 

 impossible it is to comprise, in the narrow framework of our partial 

 and merely temporary systems, the majestic series of evolving events 

 that have filled up the untold ages which have left their traces in- 

 scribed even on the crust of this little globe of earth. 



vol. xix. — FART I. 



