134 HOLLAND : GEOLOGY OF THE NEIGHBOURHOOD OF SALEM. 



into nodules and bands {schlieren) during the early stages of 

 consolidation of the rock. 1 Besides the larger nodules, such as 

 those at one time worked by the Porto Novo Iron Company, chro- 

 mite occurs as isolated granules with magnetite disseminated through 

 the dunite. 



We now know of a large number of occurrences of similar 



The prevalence of peridotites in the Madras Presidency, Mysore 



magnesite. anc j Q 00r g } an( j j n a |j cases so far examined 



magnesite, occurring in veins, constitutes the chief product of 

 secondary change in the dunite, whilst serpentine occurs only in 

 comparatively very small quantities. In view of the fact that the 

 latter mineral (serpentine) is the most abundant secondary product 

 in the better-known occurrences of peridotite in Europeand America, 

 this remarkable abundance of magnesite in the Madras Presidency 

 becomes a feature which deserves special consideration. 



There is one fact which on further investigation may point 

 to the correct explanation of this abundance 



Origin of the magnesite. 



of magnesite; most, if not all, the peridotite 

 eruptions of South India are accompanied by masses and veins of 

 pure white quartz, which always contain considerable quantities of 

 liquid carbonic acid. The constancy of this association of peridotite 

 with pure quartz suggests a genetic relationship between the two, 

 and it is not unlikely that the quartz is the end-product of the erup- 

 tion in each case. 2 If, now, the carbonic acid is present in large 



1 For summary of facts concerning chromite deposits, see J. H. L. Vogt's " Beitrage 

 zur genetischen Classification der durch magmatischen Differentiations-processe und der durch 

 Pneumatolyse enstandenen Erzvorkommen : Ausscheidungen von Chromeisenerz in 

 Peridotiten." Zeitschr.fiir. prakiiscke Geologie, 1893, p. 268, and 1894, P« 382. 



2 There appears to be no theoretical objection to this idea of peridotite and quartz 

 being derived from the same magma. The protoxides of iron and magnesia, which are so 

 abundant in the peridotites, require a small amount of silica to produce olivine, and, in the 

 absence of alumina and other bases, the excess of silica must crystallize as free quartz; 

 this, in fact, only shows how very imperfectly our ordinary classification by silica percentage 

 expresses the natural genetic relationships between our different types of eruptive rocks, for 

 the peridotite and quartz would ordinarily be placed at opposite extremes of the eruptive list. 



( 32 ) 



