263 



DESCRIPTION OF THE ROCKS. 



We recognize two principal divisions in the present group the diorites 

 and the andesites. In chemical composition these two groups resemble 

 each other. The essential difference between them is one of texture. The 

 typical diorites are holo-crystalline and granitic ; the typical andesites are 

 semi-crystalline and trachytic. 



The textural^difterences correspond very closely with differences in mode 

 of occurrence. The diorites usually occur as intrusive masses ; the andesites, 

 as lava flows. This difference in mode ot occurrence is, however, not 

 always observed ; the margins of intrusive masses often assume an andesitic 

 character and the centres of massive lava flows a dioritic character. It is, 

 therefore, impossible to frame any rigid definitions of the two groups. We 

 will consider first of all the rocks of the dioritic and afterwards those of 

 the andesitic group. If we fix our attention on mineralogical composition 

 we observe that the diorites may be subdivided according to the nature of 

 the dominant ferro-magnesian constituents augite, enstatite (hypersthene), 

 hornblende and biotite. Two or more of these constituents are frequently 

 present. Hence, in addition to the augite-diorites, enstatite-diorites, 

 hornblende-diorites and mica-diorites we have such varieties as enstatite- 

 augite-diorites and hornblende-mica-diorites. 



Prof. ZIRKEL^ describes a quartz-diorite from Arran. It is associated 

 with Lower Carboniferous Sandstone and occurs on the left-hand side of 

 the road to Shedog as it crosses the watershed separating Glen Shirrag from 

 Glen Laodh. The rock is described by ZIRKEL as a fine-grained mixture of 

 white felspar and greenish black hornblende Avith some quartz, which, 

 however, can only te recognized by the use of the microscope. Plagioclase 

 dominates over orthoclase ; green epidote surrounds the hornblende in 

 places, the quartz contains fluid inclusions; magnetite and apatite are 

 present. The rock is holo-crystalline. 



A slide prepared from a specimen ot this rock given to the author by 

 Mr. MIERS contains in addition to the above-mentioned constituents both 

 augite and biotite. The hornblende is green. It is both idiomorphic and 

 allotriomorphic. The idiomorphic hornblende shows in cross-sections not 

 only the clinopinacoid and prism faces but also the orthopinacoid. The 

 allotriomorphic hornblende occurs in ophitic plates penetrated by the 

 lath-shaped felspar sections. Both varieties are found in one and the 

 same slide. The felspar occurs in lath-shaped sections and does not 

 as a rule show multiple twinning. Quartz fills up the angular spaces 

 between the felspar sections. The augite occurs in colourless crystals or 

 grains. It shows characteristic cleavages and optical characters (maximum 

 extinction observed 35). The biotite occurs sparingly. It is very 



(1) Z.D.G.O., 1871, p. 30. 



