266 



analyses points to the conclusion that the pyroxene is a lime-magnesia- 

 bisilicate (diopside). 



I. II. 



SiO 54-09 52-47 



Al.,6 15-02 12-15 



Fe.,0 4-12 3-47 



FeO ' 5-15 5-23 



CaO 7-72 9-71 



MgO 7-28 9-94 



Na..O 1-99 2-81 



Iv.,6 3-55 2-26 



HlO} 49 ( 1-62 



CO, j \ -54 



100-41 100-20 



I. Diorite containing little or no pyroxene ; base of Ben an Fhurian. (TEALL.) 

 II. Hornblende-augite-diorite, intrusive in clolomitk' limestone, near Inchnadampf. 



In addition to the rocks of a more or less decided granitic texture we 

 iind others, in the same neighbourhood, of a markedly trachytic character. 

 These will be referred to when we speak of the andesites. They are 

 distinguished from the diorites by their markedly porphyritic character 

 and by the occurrence of two distinct generations of felspar. 



One very interesting feature which a comparison of the different 

 varieties of diorite brings out is the variation in the state of crystallization 

 in the felspathic matrix. In certain compact rocks the individual felspars 

 are minute and give more or less lath-shaped sections. It is impossible to 

 determine the precise character of the original ground-mass in consequence 

 of alteration, but it evidently approximated to that of the andesites. In 

 the medium-grained rocks the micro-structure of the felspathic matrix is 

 thoroughly granitic. Where the felspathic matrix is very fine in texture 

 the rock assumes a porphyritic character, when only one generation of 

 felspars is present, in consequence of the hornblende crystals being larger 

 than the individuals of the matrix ; where the felspathic matrix is coarse 

 the porphyritic character is not noticeable. In both cases the hornblende 

 is idiomorphic. As there has been no recurrence of phase in the process 

 of consolidation the rocks with a fine-grained felspathic matrix would not 

 be porphyritic in the sense in which that term is used by ROSENBUSCH. 



A large mass of diorite is exposed in Glen Tilt, in Perthshire. The 

 author possesses two specimens from this mass; one resembles a grey 

 granite in aspect except that the hornblende crystals are fairly conspicuous; 

 the other is a dark rock evidently containing more hornblende than the 

 former, moreover the large cleavage surfaces of the hornblende show 

 lustre mottling. 



The grey diorite is composed of hornblende, felspar, biotite, quartz, 

 apatite, sphene and iron-ores. The hornblende occurs in crystals and 

 grains. It appears to be for the most part idiomorphic with respect to the 

 felspar. The colour of the hornblende is variable, either some shade of brown 

 or green. The felspar is idiomorphic with respect to the quartz. Twinning 

 on the albite, pericline and Carlsbad types may be recognized. Much of 



