DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 149 



greatest area being in the groups of the Eed Hills of Skye, between Loch Sligachan and 

 Strath. 



Microgranite. — This name is applied to certain intrusive masses, which macro- 

 scopically may be classed with the quartz-porphyries and felsites, but which micro- 

 scopically are found to possess a holocrystalline granitic ground-mass of quartz 

 and orthoclase, through which are scattered porphyritic crystals of the same two 

 minerals, sometimes also with plagioclase, augite, magnetite, or apatite. Rocks of this 

 type do not appear to be abundant. They occur as dykes and bosses, but occasionally 

 also as sheets. I have collected them from Skye, Rum, and Ardnamurchan. 



Granite. — That there are true granites among the acid rocks of the Tertiary volcanic 

 series can no longer be doubted. As in their macroscopic characters the more coarsely 

 crystalline granophyres are not to be distinguished from granites, and, as their dark 

 ferro-magnesian constituent is generally hornblende, they were called by the older petro- 

 graphers "syenite"; that is, granite with hornblende instead of mica. In many of the 

 granophyres, the microscope reveals transitional stages to granite. The peculiar micro- 

 pegmatitic ground-mass may be observed so reduced in amount as only to appear here 

 and there between the other minerals which are grouped in a granitic structure. From 

 this condition, one step further carries us into a true granite, from which all trace of the 

 granophyric character has disappeared. Such gradations may be traced even within short 

 distances in the same boss of rock. Thus, portions of the interior of the boss of Beinn- 

 an-Dubhaich, Skye, possess a thoroughly granitic arrangement of their component 

 minerals, while a specimen taken from near the edge on the shore of Camas Malag, shows 

 the appearance of the granophyric ground-mass. But, though the large bosses are 

 usually somewhat coarsely crystalline in the centre, and tend to assume finer felsitic 

 textures around their borders, as was observed long ago by Oeynhausen and Von 

 Dechen,* the granitic structure is sometimes exhibited even at the very edge, and not 

 only so, but in the dykes that protrude from the bosses into the surrounding rocks. 

 Thus the Beinn-an-Dubhaich mass, at its margin on Camas Malag, sends a vein into the 

 surrounding limestone, but though more close-grained than the main body of the rock, 

 this vein is neither felsitic nor granophyric, but truly granitic in structure. 



So far as I have observed, the true granites contain a brown mica and also a little 

 hornblende, both visible to the naked eye, but generally somewhat decomposed. These 

 rocks are thus hornblende-biotite-granites (amphibole-granitites of Rosenbusch). They 

 may be defined as medium-grained aggregates of quartz, orthoclase (also plagioclase), 

 biotite, and hornblende, with sometimes magnetite, apatite, epidote and zircon. Dr 

 Hatch informs me that he finds that in some instances (Beinn-an-Dubhaich) the 

 quartz contains minute inclusions (glass ?), bearing immovable bubbles with strongly 

 marked contours ; while in others (Beinn-na-Chro, Skye), this mineral is full of liquid 

 inclusions with bubbles, sometimes vibratile, sometimes fixed. He remarks that the 

 quartz and felspar have consolidated almost simultaneously, but that in some instances 



* Karsten's Archiv, i. p. 89. 

 VOL. XXXV. PAKT 2. U 



