542 B. K K Wyllie & A . Scott— Flutonics of Garaba I Hill 



formation of an amphibole much richer in the oxides common to 

 brown hornblende, accompanied by the separation of the bisilicates 

 as the crystallites. There is very little difference between the optical 

 properties of the two aniphiboles. The extinctions are the same, as 

 well as the positions of maximum and minimum absorption, the only 

 difference being in the nature of the absorbed rays. 



The "Newer Igneous Rocks". 



The series of plutonic rocks which has been intruded into the 

 Highland schists after the latter had been foliated are included under 

 the general name of the " IS'ewer Igneous Rocks ". These intrusions, 

 which are probably of Lower Old Red Sandstone age, show consider- 

 able lithological similarities. Granite usually composes the greater 

 part, but diorites are of common occurrence, while ultrabasic rocks, 

 on the other hand, are somewhat scarce. The only ultrabasic rocks 

 which have been described are the peridotites of Garabal Hill, the 

 kentallenites of Argyll,' the scyelites of the Moine Gneiss regions,^ 

 and the peridotites of the Coyles and Glen Doll,^ each of which is 

 accompanied by picrites. It is only in the last-named localities that 

 we have a suite of rocks at all resembling those we have described. 

 This includes an olivirie-enstatite rock (saxonite), an enstatite- 

 picrite, a poiplij^-itic diorite with green hoinblende, and some acid 

 diorites. This is apparently a series of intrusions of decreasing 

 basicity, with some rocks formed by amalgamation due to the later 

 intrusions. A peridotite which occurs along with scyelite in Ross- 

 shire seems to resemble the Garabal Hill wehrlite, but the scyelite 

 itself, as well as kentallenite, are totally unlike anything occurring 

 elsewhere, and seem to be more or less abnormal. 



Wherever ultrabasic rocks occur in the Highlands they are clearly 

 the earliest intrusion, and are usually followed by diorite and finally 

 tonalite and granite. That some time elajised between the various 

 intrusions is certain, as all the evidence goes to prove that the peridotites 

 were consolidated long before the diorites came up, and the latter 

 had obviously crystallized before the granite intruded. Another 

 significant feature is the absence of intermediate types. Thus at 

 Garabal Hill we have nothing between the felspathic davainite, 

 representing the least basic of the peridotites and with 43 per cent 

 silica, and the hornblendites, the most basic diorite, with 49-52 per 

 cent silica. Again, in Glen Doll, there is no rock intermediate 

 between a picrite, containing a good deal of olivine, and a fairly 

 normal diorite. A similar gap exists between the tonalite and diorite, 

 for the acid diorite of Garabal Hill has about 54 per cent of silica, 

 while the tonalite has never less than 62 per cent. Hence we have 

 clearly three separate series of rocks, each more or less complete in 

 itself and each showing not only the normal rock with slight 



^ Flett in Geology of Oban and Dalmally (Mem. Geol. Surv. Scotland), 1908, 

 pp. 82-109. 



- Flett in Geology of Ben Wyvis, etc. (Mem. Geol. Surv. Scotland), 1912, 

 pp. 126-9. 



' Barrow in Geology of Braemar, etc. (Mem. Geol. Surv. Scotland), 1912, 

 pp. 73-83. 



