CLASSIFICATION OF THE ROCKS. 133 



the usual 1 differences in structure due to more rapid cooling (Speci- 

 mens Nos. 1 1*933 and 1 1*934). Sections taken from near the selvage 

 of a three-inch dyke (11*934) show very prettily the rapid transition 

 from the black opaque, probably glassy, matrix to the pilotaxitic 

 base, in which the porphyritic crystals of pyroxene and plagioclase 

 are imbedded. 



4b. — The Magnesian Series. 

 The peculiar rocks forming the so-called "Chalk hills" were 

 mapped by Messrs. King and Foote, and their field characters de- 

 scribed with accurate detail in their memoir published in 1864. 8 

 The prevalence of peridotites within this area was established by me 

 in 1892, 8 the detection of these as the primary rocks leading to a 

 simpler explanation for the origin of the secondary magnesite in this 

 and in the many similar occurrences of peridotite subsequently found 

 in South India. 



The prevailing type of peridotite in the Chalk hills is olivine- 



rock, containing, like the similar dunite of New 



Prevalent ftfrms. . , 



Zealand, quantities of magnetite and chromite 

 with sometimes enstatite. It is accompanied by dyke-like masses of 

 mica-augite picrites which sometimes show very fine poikilitic 

 structures. 



Secondary alteration of these rocks has resulted in the form- 

 ation of magnesite, chalcedony, serpentine (prin- 



Alteration products. .»«_'. * e c ■• » . 



cipally in the form of picrolite) and talc. The 

 white appearance of the area due to the large quantities of magne- 

 site gave rise to the ill-chosen name " Chalk hills." 



The chromite occurring in the dunites of the " Chalk hills " is 

 as usual in these rocks very irregular in its dis- 

 tribution, a circumstance due to its segregation 



1 See Rec , Geol. Surv., Ind., Vol. XXX, p. 36. 



2 Op. tit., p. 312. 



3 Rec, Geol. Surv., Ind., Vol. XXV, pp. 143, 144. 



( 31 x 



Chromite. 



