OLDER PERIDOTITES OF SCOTLAND. 395 



dunites, Iherzolites, and picrites, we have the felspathic rocks which 

 must be assigned to the troctolites (anorthite-olivine rocks) and the 

 eiicrites (anorthite-augite rocks). 



The troctolites (forellensteins) in all cases exhibit intergrowths 

 of felspar-crystals, forming a mass through which single grains or 

 groups of grains of olivine are irregularly distributed (see PL XIII. 

 fig. 5). Except in the perfectly fresh and unaltered condition of 

 the minerals of which they are composed, and in their finer grain, 

 these rocks exactly resemble in structure the well-known forellen- 

 stein of Yolpersdorf. 



The eucrites (anorthite-augite rocks) exhibit both the granitic 

 and granulitic types of structure. An interesting example of the 

 latter variety is illustrated in PI. XIII. fig. 6. 



The mass of basic and ultra-basic rocks in the island of Rum, 

 which covers an area of from eight to ten square miles, and rises 

 into a number of mountains varying from 2000 to 3000 feet in 

 height, is made up principally of the minerals anorthite (or a felspar 

 closely approximating in composition to that species), augite, and 

 olivine. When all three are present, as is most frequently the case, 

 we have an olivine-gabbro ; when the first disappears we get a 

 picrite, when the second is wanting we have a troctolite, and when 

 the third is absent a eucrite. When both the first and second are 

 wanting the dunites are formed ; and the addition of the less abun- 

 dant minerals, the enstatites, the biotites, and the spinellids, gives 

 rise to Iherzolites and other varieties. All these forms are found 

 passing into one another by the most insensible gradations, and it 

 would be possible, though, I think, most inexpedient, to propose 

 names for other cuiious mineral combinations which occur here. 



The Shiant Isles ofi'er perfectly similar examples of transitions 

 between these different types of basic and ultra-basic rocks. 



Part II. 



The Paljeozoic Peridotites and Allied Rocks. 



So far as is at present known, there are no peridotites of Mesozoic 

 age in Scotland. The numerous masses of more or less altered 

 ultra-basic rocks which are found scattered about the country, appear 

 to have been formed during the Palaeozoic epochs ; but some of them 

 may be of Archaean age. Certain of these old peridotites can, how- 

 ever, be proved to be of the age of the Old Red Sandstone and the 

 Carboniferous. 



At first sight the Palaeozoic peridotites of Scotland appear to 

 present the most striking contrast in their characters with those 

 which we have been describing as belonging to the Tertiary epoch. 

 But the more carefuU}- we study these rocks, the more distinctly is 

 it seen that the differences between the older and younger rock- 

 masses are not essential but accidental ones, — the result of al- 

 terations which have taken place during the enormous periods of 



