198 



Mr. S. Allport and Prof. T. G. Bonney. [June 6, 



analysed by Mr. F. W. Clarke,* from American localities. In fact, 

 these contact-micas are either lepidomelanes or haughtonites, varietal 

 names which are applied to iron micas, according as there is a pre- 

 dominance of Fe 2 3 or FeO.f 



The same analyses indicate that if a normal clay were converted 

 into a quartz-mica rock, supposing all the alkalies present to be 

 taken up in forming mica, a considerable proportion of alumina, and 

 the larger part of the silica (something like 50 per cent.) would be 

 left unused. If the alumina amounted to 11 per cent, of the whole, 

 about tr6 of the silica would be required for andalusite ; this would 

 leave not less than 43 per cent, of the silica free to form quartz. 

 Thus from a normal clay, as the result of contact metamorphism, 

 a rock is produced containing not less than 50 per cent, of free 

 quartz. 



(6.) After what has been said above, the history of a sandstone 

 may be dismissed in few words. Almost all sandstones contain a 

 certain amount of mud and silt. From these, both mica and sub- 

 silicates of alumina would form as before ; but in this case the free 

 silica would very commonly be deposited on the pre-existing granules 

 or grains of quartz, probably in crystalline continuity with them, so 

 that in many cases the fragmental aspect of the rock would be 

 obliterated and its structure would resemble that of the quartz layers 

 in the true crystalline schists. 



In the above cases, if the original rock contained a lower proportion 

 of alkalies and a fair one of lime, as well as some magnesia, we might 

 expect minerals of the pyroxene group to be formed. If, however, 

 felspar was present in fragments, and its constituents entered into new 

 combinations, a white mica might be produced. A fragment of normal 

 orthoclase, in forming muscovite, would liberate at least four-tenths 

 of its silica,^ and if it had lost some of its alkaline constituents by 

 decomposition, it would be converted into a roughly equal mixture of 

 quartz and muscovite. 



Somewhat similar changes would occur in the case of fragments of 

 an acid glass, which might be expected to produce, as the result of 

 contact metamorphism, a microcrystalline mixture of quartz and 

 white mica, while the alteration of a basaltic rock§ would give rise to 

 pyroxenic minerals, epidote, probably some biotite or chlorite, with a 

 smaller proportion of quartz. 



On examining some of the slides in Mr. Allport's collection from 



* ' Amer. Journ. Sci.,' vol. 34, 1887, p. 131. 



f Mr. Allport (' Quart. Journ. G-eol. Soc.,' vol. 32, p. 417) found that lirhia was 

 present in Cornish specimens of the brown contact-mica, but probably this is not an 

 essential constituent. — T. G-. B. 



J Bonney, ' Quart. Journ. Greol. Soc.,' vol. 44, p. 37. 



§ Allport, ' Quart. Journ. Greol. Soc.,' vol. 32, p. 407, &c. 



