600 Geological Society. 



November 21st.— J. J. Teall, Esq., M.A., F.R.S., President, 

 in the Chair. 



The following communication was read : — 



1. ' A Monchiquite from Mount Girnar, Junagarh (Kathiawar).' 

 By John William Evans, D.Sc, LL.B., E.G.S. 



After a brief account of the rocks of the monchiquite-type, in 

 which ferromagnesian silicates are embedded in an isotropic matrix 

 with the chemical constitution of analcime, the author describes 

 an example from Mount Girnar, where it is associated with a 

 nepheline-syenite intrusive in a mica-augite-diorite. 



The most striking feature of this rock is the occurrence of 

 colourless spheres of various sizes up to about 1 mm. in diameter. 

 The rest of the rock is mainly composed of a hornblende of the 

 barkevikite-type ; a pale green augite is also present. Both the 

 spherical spaces and the interstices between the ferromagnesian 

 silicates are usually filled with an isotropic material which has the 

 composition and most of the physical properties of analcime. This 

 material does not, however, show the anomalous double-refraction 

 which is characteristic of that mineral, nor has it any crystalline 

 outlines, being simply an allotriomorphic glass-like groundmass. It 

 contains a large number of acicular inclusions, most of which do not 

 affect polarized light : they exhibit a parallel arrangement in one or 

 more directions, and appear to indicate a high degree of symmetry. 

 Cleavage -cracks with similar orientation may be occasionally 

 observed. As it is clearly a crystalline body, its isotropic nature 

 refers it to the cubic system, and its identity with analcime may 

 be considered proved. It is evident that this mineral, growing 

 outward from different centres, has formed the spherical spaces by 

 pushing aside the previously crystallized minerals until they came 

 into contact one with the other, and has afterwards crystallized in 

 the interstices between them. 



The presence of a groundmass of analcime (or one having the 

 same composition) in all the members of the widely distributed 

 monchiquite-group of rocks implies the occurrence in different 

 localities of a residuary magma of uniform composition, which 

 remains liquid after the other constituents of the rock have 

 crystallized out. Analcime must, therefore, represent an eutectic 

 compound. If the cooling were sufficiently rapid the magma 

 would consolidate as a glass, as may be the case with some 

 monchiquites. On the other hand, where such a magma has 

 separated and cooled slowly enough, a nepheline-syenite will be 

 formed. 



At some points the analcime in the spheres and in the interstices 

 has become decomposed into alkali-felspars and nepheline, as in the 

 pseudo-leucites of Dr. Hussak, so that in these places the rock 

 might be described as a hornblende-tinguaite. In other parts much 

 of the analcime has passed into cancrinite. 



The presence of a mineral of the eudialyte-eucolite group is also 

 noticed. 



