Reviews — Dr. J. Petersen on Cheviot Rocks. 229 



felspars were isolated and analyzed, and my determination of them 

 as labradorite is confirmed. Their specific gravity lies between 

 2-70 and 2-65. 



The constituents of the ground-mass comprise microlites of 

 pyroxene, felspar, small crystals of hematite and magnetite, together 

 with globulites, longulites, belonites, and other devitrification pro- 

 ducts. 



The ultimate base is perfectly isotropic ; but after heating a thin 

 slice on platinum foil, it gives a distinct reaction with polarized light. 

 Inasmuch as the natural glass contains water, we appear to have in 

 this most interesting experiment a proof of the conclusion that the 

 passage from the glassy to the crystalline condition is at any rate in 

 certain instances dependent on a loss of water. 



The glassy base was isolated by the author and analyzed by E. 

 Ebert. Sp. gr. 2437. 



SiOs 



66-25 



AUO3 



.. ... 13-59 



FeoOs 



3'11 



Cab 



2-75 



MgO 



0-28 



KoO 



4-95' 



NaoO 



2-25 



H3O 



5-89 



99-07 



Some interesting chemical observations were made upon the red 

 veins (see Geol. Mag. Decade 11. Vol. X. p. 106). When these veins 

 are wide (1 mm.), the central portion is colourless ; the marginal poi'- 

 tion is always a deep red. The specific gravity of the clear substance is 

 2-440, and the amount of water 2-89 percent. ; -specific gravity of the 

 red substance 2-071, and amount of water 7-07 per cent. Qualitative 

 analysis in both cases furnished only silica, ferric oxide and water. 

 The clear substance is therefore chalcedony ; the red substance opal. 



Microscopic examination shows that the veins do not arise through 

 the alteration of the surrounding rock, which is perfectly fresh, eveu 

 up to the junction. 



The altered rocks or normal porphyrites are then referred to. Some 

 of these are undoubtedly the altered representatives of rocks similar 

 in mineralogical composition, to the " pitchstone-porphyrite " (my 

 hypersthene-andesite). I do not think however that they were identi- 

 cal in composition with this rock. My own view is that they (I am 

 speaking now only of those rocks which I regard as having been in 

 the first instance hypersthene-andesites) differed originally in the 

 physical condition or chemical composition of the ground-mass by 

 reason of which they were more readily affected by the agents of 

 alteration. The author does not seem to be aware that the fresh 

 rock is completely subordinate to an immense development of normal 

 porphyrite which shows traces of alteration throughout its entire 

 mass. Had the hypersthene-andesite been equally liable to altera- 

 tion, I do not think it would have been preserved. The 

 green mineral which results from the alteration of the pyroxene 



