

SUMMARY OF RESULTS. 147 



by the action of carbonic acid at high temperatures upon the 

 original rock? of the tl Chalk hills." I pointed out, however, in 1892 

 that the original rocks of the Chalk hills were peridotites with dunite 

 as a predominant type, not metamorphic rocks. This was confirmed 

 in 1896 by Mr. Middlemiss who, however, regarded the magnesite 

 as the result of the alteration of serpentine previously formed by 

 hydration of the olivine rocks. This additional revision of the 

 original explanation appears to be unwarranted by the facts : serpen- 

 tine exists in small quantities, but there are no signs of general 

 hydration ; the magnesite forms veins of all sizes down to microscopic 

 developments along the characteristically irregular cracks through 

 the olivine crystals, and the presence of much liquid carbonic acid 

 in the associated masses of white quartz considerably strengthens the 

 theory first suggested by King and Foote (pp. 32 — 35). 



There are many such examples of peridotites in South India 

 largely altered to magnesite, and they are often, if not always, 

 accompanied by masses of white quartz containing liquid carbonic 

 acid. The association of two such extremes — dunite and quartz — 

 is far too frequent to be merely fortuitous ; and it is not unlikely 

 that the two are genetic relatives, the quartz representing the 

 siliceous end-product of the eruption, which, in the absence of 

 alumina and alkalies, must consolidate as simple quartz instead of 

 forming alumino-alkaline silicates. Elsewhere evidence has been 

 cited which indicates that in a crystallizing igneous magma the 

 water excluded to the final stages of consolidation may attack and 

 decompose an early-formed ferro-magnesian silicate ( Quart. Journ. 

 Geo/. Soc, LIII (1897), 413). In the present instance the water may 

 have given rise to the formation of picrolite, whilst carbonic acid 

 similarly excluded to the siliceous residue, has acted in an analogous 

 manner on the separated olivine, forming magnesite by its action on 

 the pre-formed ferromagnesian silicate. 



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