CHEMICAL EXAMINATION OF ROCKS AND MINERALS. 75 



meratcd together and intermixed with fragments of the rock more or less 

 decomposed. The crystals of pyrites which occur in the residue arc 

 always cubes with perfectly sharp edges and angles, the latter being some- 

 times truncated. There is every reason to suppose that these crystals were 

 formed subsequently to the interval which gave birth to the limestone. 

 Some of them were found in the midst of the quartz debris ; and one of them 

 consisted of a kind of twin parallel to the faces of the cube, the two halves 

 being, however, separated by a portion of quartz ; one half had its edges 

 truncated and the other not. They had thus submitted to the conditions 

 imposed by the medium in the midst of which they were developed. 



The following are the results of an analysis of a specimen of this lime- 

 stone : — 



Carbonate of lime 75*987 



Carbonate of magnesia - 986 



Peroxide of iron and alumina 1*583 



Residue consisting of chlorite, magnetic, and com- \ 01.35(5 



mon iron pyrites and debris of hyaline quartz. . J 



99-912 



It results from these observations that this metamorphic limestone should 

 be regarded as derived from some of the materials of the schist above 

 mentioned. For we may, so to speak, follow the passage of the mica-schist 

 from the point where it does not effervesce with acids, into metamorphic 

 limestone still containing all the essential parts of the schist. If we suppose 

 that this limestone had been subjected to a high temperature, the quartz 

 should have combined with the bases. The crystals of pyrites disseminated 

 through the mass, as well as the position which they occupy, suggest an 

 argument of a similar kind. 



7. Gweedore Metamorphic Limestone. — This rock is found associated with 

 mica-schist resting on granite near Gweedore River ; isolated patches of the 

 limestone occasionally rest on the granite, and sometimes alternate with mica- 

 slate. This limestone is saccharoidal, of an aquamarine tint, which is due to 

 the mass of small angular fragments of a green mineral interspersed through 

 it. This mineral often serves as a nucleus to a crystal of carbonate of lime, 

 aud is accompanied by small sand-like crystals of idocrase and garnet. 

 Large crystals of garnet are also found in abundance ; and from the way in 

 which they are deposited, the rock has a stratified appearance. The faces 

 of the crystals are more or less eaten away, as if they had been weathered. 



Treated for some time with very dilute hydrochloric acid, this rock gave 

 in 100 parts the following results: — 



' Garnets, idocrase, 

 „ . . . ,. „ and green mineral . . 17*16^1 



Residue consisting of j Amorp f lous silica .... 6 - 3 >23*360 



Alumina 0*1 7 J 



Carbonate of lime 75*250 



Carbonate of magnesia 0*610 



Alumina and oxide of iron soluble in acids 0*512 



Water (undetermined) 



99*732 



If a fragment of this garnet limestone be left in very dilute hydrochloric 

 acid until the whole of the carbonate of lime be removed, the garnets will 

 be found imbedded in nearly pure amorphous silica, which readily dissolves 

 in a weak solution of caustic potash. This siliceous paste is obviously the 



