426 J. A. PHILLIPS ON THE CHEMICAL AND MINERALOGICAL 



cent, of silica, while Nos. III., and IV. have received respectively 

 an increase of 1*35 and *77 per cent, of that body. 



The proportion of silica contained in all the different portions of 

 the same crystalline rock-mass, however, is never rigorously con- 

 stant ; and consequently, taking into consideration the more or less 

 altered state of the various specimens operated on, they may be 

 regarded as not having materially changed in this respect. 



After a careful examination of the unaltered rock No. I., free 

 quartz could be distinguished in very small quantity only, whereas 

 in Nos. II., III., and IY. quartz granules are conspicuously abun- 

 dant ; it therefore appears not improbable that the presence of this 

 mineral may be to some extent due to the crystallization of pro- 

 gressively dissociated silica, although the greater proportion of it 

 may have perhaps formed one of the crystalline constituents of the 

 original rock. 



It is worthy of notice that specimens, in the composition of which 

 quartz occupies a conspicuous proportion, do not contain a larger 

 amount of silica than those in which its presence can be barely 

 distinguished by the aid of the microscope. It is likewise to be 

 observed that a few only of the quartz granules contain liquid- 

 cavities, and that under a power magnifying 675 linear the latter 

 are occasionally seen to enclose bubbles in a state of active motion. 



On referring to the analyses it will be observed that the propor- 

 tion of alkalies in the different specimens does not materially 

 differ ; this substantially agrees with the observations of Ebelmen, 

 who found that the removal of potash and soda from decomposed 

 pyroxenic rocks was much less complete than that of lime and 

 magnesia*. 



This rock, which in chemical composition is intermediate between 

 the basic and acidic groups, corresponds, when almost free from 

 quartz, very closely with the trachydolerites of Abich, who includes 

 under this name not only augitic varieties, but also those containing 

 hornblende alone. In its ordinary condition it is a quartziferous 

 diorite. 



The foliated deposit previously referred to as containing oxide of 

 manganese, occurs as a stratified layer at the top of the more 

 westerly quarry belonging to Messrs. Kneeshaw, Lupton, and Co. 

 This band, which is moderately coherent, is of a light reddish brown 

 colour, but is considerably less hard than the crystalline rock upon 

 which it rests. Under the microscope thin sections are seen to 

 consist of an amorphous base stained in places by ferric oxide, and 

 enclosing patches of crystalline chlorite, together with fragmentary 

 crystals of hornblende, of which the outlines gradually merge into 

 the base ; it also contains a little pyrites and a few minute sphe- 

 roidal bodies of a brown colour. 



A specimen taken from this band, analyzed in duplicate, afforded 

 the following results (sp. gr. 2*80) : — 



* Ebelmen ' Eecueil des Travaux,' ii. 1-79, quoted by T. S. Hunt, ' Chemical 

 and Geological Essays,' p. 101. 



