STRATIGRAPHICAL ELEMENTS: CRYSTALLINES, ETC. 83 



saussuritic. Probably the two boulders are different portions of a 

 very coarsely banded hornblende-gneiss. 1 Garnet is common in these 

 rocks, associated with augite as referred to above. It is in irregular 

 grains, sometimes shewing crystal outlines. It also occurs without 

 augite in some of the rocks. It is pink in colour and traversed 

 by irregular cracks, limpid and clear and without inclusions. Quartz 

 is not common. There are only a very few examples of it in 

 small grains, lying along with the felspar and possibly having 

 arisen secondarily by crushing and metamorphism of the felspar. 

 Iron ore is sparsely represented. In addition to the above mi- 

 nerals, hypersthene and olivine (?) are doubtfully present in one 

 rock ^ftf. 



The above gives an idea of the general mineral components of 

 this series of rocks. As regards their structure, they are sometimes 

 massive crystalline rocks in which no foliation can be detected, and 

 on the other hand they are often foliated very distinctly. That these 

 rocks form a connected series, the individuals of which pass into 

 each other, seems to me indisputable ; and it does not seem improba- 

 ble, if one may make a guess founded on such scanty data, that they 

 represent a thoroughly holocrystalline plutonic massif or plexus, 

 originally consisting essentially of plagioclase and augite, with which 

 the basic dykes, already described, are in deep-seated connection. 

 The modification of them into foliated rocks with augen and lenti- 

 cular-tabular structures, and the development of hornblende from 

 augite, also seem to be plutonic effects of dynamic metamorphism, 

 corresponding to the effects we have already traced at the margins 

 of the dyke rocks. 



No. ^| f stands alone among these boulders as a fine-grained 

 dolerite, composed of lath-shaped plagioclases, pale brown augite in 

 very small rounded grains, and magnetite. 



1 Since writing the above I have made the acquaintance near Salem, Madras Presi- 

 dency, of a set of hornblende-garnet rocks which compose whole ridges. They also and 

 an ultrabasic olivine bearing rock resembling that to be presently described appear in- 

 timately connected with serpentinised dunites. 



( 83 ) 



