'710 W. N. Benson — Origin of Serpentine. 



iv. Though, mesh-structure serpentine (chrysolite) is usually 

 formed in regions of comparatively low pressure, and 

 antigorite under high pressure, instances occur where 

 antigorite has been formed by the recrystallization of 

 mesh-structure serpentine. This is best explained by a 

 recrystallization of the rock under an increased pressure 

 in the one orogenic epoch of igneous activity, and it is 

 unlikely that the solutions concerned in the process were 

 of meteoric origin. 



v. In some instances, after a rock had been more or less 

 completely serpentinized, coarse-grained veins of antigorite 

 and fresh glassy olivine have been formed in it, recalling 

 in some measure the occurrence of pegmatite veins in 

 granites. Such veins seem to be connected with others in 

 the invaded rocks showing minerals usually indicating 

 contact-metamorphism. 



vi. Serpentinization and carbonation have often been complete 

 at the close of the orogenic epoch during which the 

 peridotite was erupted, 

 vii. But while serpentinization thus must have followed the 

 intrusion of the peridotite after a comparatively short 

 interval, there have often (probably generally) been 

 several intervening intrusions of differentiates of the 

 magma that gave rise to the peridotite. This is shown by 

 (a) the spatial relationship of the post-peridotitic intrusive 

 masses to the development of serpentine; and, (6) alter- 

 ation of masses intrusive into the ultrabasic rocks, which 

 is of such a character as to indicate that the alteration 

 of these later masses occurred during the process of ser- 

 pentinization. 

 viii. The process of serpentinization, if explicable as here indi- 

 cated, is in some degree analogous to the process of grei- 

 senization of granitic masses. It is probable that the 

 carbonic acid at first acted as a catalyser, but subsequently, 

 under cooler conditions, remained in combination with the 

 magnesia. 



We now proceed to give the evidence for these several 

 statements : — 



i. Holland's investigations (1899a) of the magnesian 

 rocks of the Salem District, Madras, show that they are 

 locally altered to magnesite, and that "most, if not all 

 the peridotites of Southern 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. ' ' 

 The discussion of (iii) will show that similar action of 



