80 JF. M. HvtcMngs— Rod's of Great Whin Sill. 



the idocrase. No spinels are seen except as enclosures in those two 

 minerals. The greater portion of them are of a good deep-green 

 colour, and exactly resemble those seen in altered limestone of 

 bombs from Somma ; there are also colourless and pale reddish- 

 brown crystals. 



If we take several thin sections of specimens from this occurrence 

 and average, as it were, the results of microscopic examination, we 

 find that a large proportion of the rock consists again of a base or 

 groundmass, which varies greatly in its texture and fineness of 

 grain. Sometimes it is a close-grained, felsitic-looking mass, quite 

 cryptocrystalline, and nothing definite can be made out as to its 

 component minerals. At other parts it becomes coarser, and 

 examination with high powers seems to show that much of it is 

 quartz and felspar ; this conclusion being confirmed when we come 

 upon good-sized patches, like glassy spots in ordinary light, vidiich 

 with crossed nicols are seen to consist of well-twinned felspar 

 with sometimes more or less quartz. This groundmass may be 

 described as a calcareous adinole. In it are bedded many new 

 minerals besides garnet and idocrase. 



Wollastonite occurs at some parts in considerable abundance, 

 mainly as radiating sheaves and bunches, with sphene, epidote, and 

 recrystallized calcite. In some slides are well-developed chloritic 

 " spots," as well as others of the pale yellow-green, almost quite 

 isotropic, granular matter; and there are some which appear to 

 be cordierite in early stages of development, corresponding exactly 

 with similar spots seen to occur together with undoubted cordierite 

 in other contact-rocks. 



A large hand-specimen of this rock from Falcon Glints I have 

 analyzed. It contains : — 



Silica ... ... 53-80 percent. 



Alumina 20-25 ,, 



Ferric Oxide 8-15 ,, 



Lime 3-27 ,, 



Magnesia 3-02 ,, 



I'lf^ 2-.^J " [B-se 



ooda ... ... ... ... ... 6-54 ,, j 



Water and Carbonic Acid ... ... 2-90 



100-25 

 Another occurrence of a similar calcareous adinole is found at 

 Sneblazes. A thin section shows the same sort of groundmass. 

 No garnets or idocrase appear in the specimen examined, but there 

 is a good deal of augite in small crystals, and felspar is again seen 

 here and there. The analysis of this rock gives : — 



SiKca 50-60 percent. 



Alumina .. ... ... ... ... 20-35 



Ferric Oxide 8-30 



Lime 7-75 



Magnesia ... ... ... ... ... 2-58 



Potasli 2-39 



Soda 4-36 



Water and Carbonic Acid ... ... 3-80 



100-13 



6-75 



