CHANGES IN CERTAIN ERUPTIVE ROCKS OF NORTH WALES. 425 



felspar is, in places, so changed as to form an almost amorphous 

 base, which is rendered to a great extent opaque by a pulverulent 

 secondary product of a greyish colour; this may be kaolin, or some 

 hydrous magnesian mineral. 



Although this alteration of the felspar frequently interferes with 

 its action upon polarized light, the striation of plagioclase is some- 

 times distinctly apparent. The quartz contains minute cavities, 

 some of which appear to be full of a liquid, while others are empty; a 

 few enclose bubbles in a state of constant motion. The hornblende, 

 which is usually of a light greenish-brown colour, is moderately 

 abundant, but is seldom very distinctly dichroic *. Occasional 

 needles of apatite are present ; and felspar is sometimes observed to 

 protrude into crystalline quartz ; while patches of the felspathic base 

 are often enclosed in that mineral. 



III. From the same Quarry as No. II. — The texture of this rock 

 is of about the same degree of fineness as that of No. I. ; but it is 

 lighter in colour, and has evidently undergone more extensive altera- 

 tion. Under the microscope it is seen to consist of a transparent 

 base, containing grains of quartz, and enclosing numerous indistinct 

 dusky forms, indicating that crystals of felspar of nearly the size of 

 those occurring in No. I. have become almost completely kaolinized. 

 In this mixture are imbedded hornblendic crystals and a little mag- 

 netite or ilmenite. 



IV. From Messrs. Brundrit and Go's, eastern Quarry. — This rock 

 closely resembles No. II., but is coarser in grain, and the felspar is 

 more extensively altered. Sections cut from specimens obtained 

 from this locality appear to contain sometimes a few minute patches 

 of an altered augitic mineral. 



The microscopical examination of thin sections of variously altered 

 specimens of this rock indicates that its metamorphism has been 

 attended by a progressive kaolin ization of felspar, while the more 

 altered varieties contain a somewhat smaller amount of the horn- 

 blendic mineral ; on the other hand patches of crystalline quartz 

 become gradually more abundant. 



Chemical analyses of the same specimens show that these changes 

 have been accompanied by the removal of lime and magnesia and 

 the formation of hydrated silicates, while the proportion of silica 

 in the altered rock is not necessarily increased. At first sight the 

 latter statement would not appear to be absolutely correct ; if, 

 however, we calculate what should be the proportion of silica in 

 rock No. I., if so changed as to contain the amounts of lime, mag- 

 nesia, and water found respectively in Nos. II., III., and IV., the 

 amount of silica would in the first case be 61 1 per cent., and in the 

 two others 61 per cent. 



It therefore becomes evident, supposing all the specimens to have 

 had originally a similar chemical composition to that of the prac- 

 tically unaltered rock No. I., that No. II. must have lost 3 per 



* The form of these crystals is seldom sufficiently perfect for complete iden- 

 tification ; but some of them are strongly dichroic, and their structure is that 

 of hornblende ; others, which are very pale in colour, are not distinctly dichroic. 



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