OLDER PEEIDOTITES OF SCOTLAND. 355 



The peridotites and their allies are of especial interest to geologists 

 on four different grounds : — First, their mode of occurrence has 

 been thought to be suggestive of their having come from deeply- 

 seated portions of the earth's crust, and they have therefore been 

 supposed to afford some indications concerning the constitution of the 

 earth's central mass. Secondly, they are the source from which many, 

 if not, indeed, all, of those very interesting rocks the serpentines have 

 been derived by hydration. Thirdly, in rocks of this class the 

 remarkable mineral, diamond, has been found m situ. And fourtJdy , 

 they exhibit many striking points of resemblance with those won- 

 derful "extra-terrestrial rocks" — the meteorites. 



The peridotites which have, up to the present time, been described 

 appear to be plutonic rocks, forming either the whole or portions of 

 intrusive masses. The enclosures in basalt and the " olivine-bombs " 

 contained in basaltic tuffs constitute apparent — but probably only 

 apparent — exceptions to this rule, for they would seem to have 

 been brought from below entangled in the lavas and scoriae with 

 which they are associated. Professor Eosenbusch is inclined to 

 regard the " limburgites " as analogous to the peridotites, and some 

 of these rocks certainly have an ultimate chemical composition sug- 

 gestive of an alliance with the ultra-basic types. But the majority 

 of the limburgites would seem to be, like the magma-basalts of 

 Boficky and Mohl, rocks in which consolidation has taken place 

 before the individualization by crystalline action of the more acid 

 constituents which form the felspar, nepheline, or leucite. 



The true peridotites have been usually regarded as belonging to 

 the older geological periods ; by some authors, indeed, they have 

 been stated to be absolutely characteristic of the Pre-Tertiary epochs. 

 In Scotland, however, we find great masses of undoubted peridotites, 

 exhibiting all the essential features of their older analogues, but 

 associated in the most intimate manner with the intrusive rock- 

 masses which I have, on a previous occasion, shown to be the central 

 cores of great volcanoes built up by successive outbursts during the 

 earlier Tertiary periods. 



But it is not alone on account of their younger geological age that 

 the peridotites and allied rocks of the Western Isles of Scotland are 

 of such extreme interest to geologists. Owing probably to their 

 comparatively recent origin, the minerals of which these rocks are 

 composed are in a remarkably fresh and unweathered condition, 

 supplying us, as I hope to show in this paper, with admirable 

 facilities for studying certain questions concerning their origin and 

 mode of development. Examined from this point of view, the Ter- 

 tiary Peridotites of Scotland furnish us with valuable suggestions 

 for the solution of many problems concerning the older rocks of the 

 same class, and their altered representatives, the serpentines, not 

 only in Scotland, but in other parts of the globe. They are also 

 important from their grand development in this district — forming, 

 as we shall see that they do, a considerable portion of great moun- 

 tain-masses, which cover considerable areas. 



