128 Geological Society : — 



May 26th.— Dr. Henry Hicks, F.R.S., President, 

 in the Chair. 



The following communications were read : — 

 1. ' On Augite-Diorites -with Micropegmatite in Southern India. 

 By Thomas H. Holland, Esq., A.E.C.S., F.G.S., Officiating Super- 

 intendent, Geological Survey of India. 



This paper deals with a series of basic dykes intersecting the 

 pyroxene-granulites and gneisses of the Madras Presidency, and 

 believed to be of the same age as the lava-flows of the Cuddipah 

 system. These dykes consist essentially of augite (near heden- 

 bergite) and a plagioclase-felspar (near labradorite), between which 

 we find masses of micropegmatitic intergrowths of felspar and 

 quartz, with a micro-miarolitic structure. Around the patches 

 of micropegmatite, chemical changes have frequently taken place 

 in the minerals of the rock. Similar occurrences of micropegmatite 

 in basic rocks have been described by J. A. Phillips and Waller 

 at Penmaenmawr, by Hill and Bonney in Charuwood Forest, by 

 Teall in the Whin Sill and in the Cheviot district, by the last- 

 named author and Harker at Carrock Fell, by Sollas at Caiiingford, 

 and by Harker in Skye. 



After discussing the chemical constitution of the rock, and of its 

 various constituents, and the relation between the micropegmatite 

 and the surrounding minerals, the author points out that three 

 methods for the formation of the micropegmatite may be conceived 

 of:— 



(a) During the primary consolidation of the magma. 

 (/3) By secondary changes induced in the rock, 

 (y) By subsequent intrusion of granophyric material into the 

 augite-plagioclase rock. 



In opposition to (y), the author points out the entire .absence of 

 granitic intrusions in the neighbourhood. He regards the absence 

 of all proofs of subaerial hydration, and the remarkable freshness of 

 the rocks as precluding the possibility of the micropegmatite having 

 been formed by secondary change. The primary origin of the micro- 

 pegmatite he believes to be proved by : — (1) The crystallographic 

 continuity of its felspar with that of the normal plagioclase of the 

 rock ; (2) the mode of occurrence of the micropegmatite, filling in 

 the angles and spaces between the augite and the plagioclase ; and 

 (3) its variation in coarseness of grain agreeing with that of the 

 remaining two constituents of the rock. 



These augite- diorites with micropegmatite are then compared with 

 the granophyric gabbros of Barnavave, Carlingford, described by 

 Prof. Sollas, to which the author maintains that his explanation 

 equally applies. He insists that the separation of the acid and basic 

 materials of a magma, which takes place so frequently on a large 

 scale, equally occurs in the midst of a consolidating mass, and com- 

 binations such as that described in the paper will be formed. He 



