ON THE VARIOLITIC ROCKS OP MONT GENE7RE. 



309 



which scattered patches occur plastered on to its western face. A 

 little to the north of this a faulted junction of the compact and the 

 porphyri tic diabase is well shown. 



The latter weathers into great 



Fig. 3. — Roclc-face showing relations of Variolitic Diabase (Jf and 

 W), Diabase Dyhe{W), Porpliyritic Diabase {V% and Tuff {T), 

 on luest slope of north ridge of Le CJienaillet. 





The spheroidal porphyritic diabase (P-') is exposed from beneath the variolitic 

 diabase (D^ and D^) and tuff* (T), and shows faulted junctions with these ; 

 a fault parallel to the ridge, and giving a nearly horizontal trace, separates 

 the spheroidal porphyritic diabase (D^) from a small patch, broken by 

 curved joints. The junction of D^ and D3is faulted. The tuff" overlies 

 the variolitic diabase B'\ and both are cut by an intrusive dyke of compact 



CllSiDBSC. 



F. Faults. 



rounded spheroids, the faces of which appear to he surfaces of cooling, 

 since the masses are often jointed within into radial prisms; the 

 master-joints, however, often cut straight through the spheroids. 



Microscopic study shows that the porphyritic diabase is composed 

 of plagioclase, augite, and some decomposition-products, in a granular 

 base that probably represents a devitrified glass. The plagioclase 

 belongs to two periods of consolidation ; there are the large saus- 

 suritic and porphyritic crystals, which are greatly corroded by the 

 ground-mass, in one specimen all the angles being rounded. These 

 crystals are traversed by veins of secondary felspar, which are in 

 optical continuity with the adjacent crystal (PI. XIII. fig. 1) even 

 when they extend a little distance beyond it. The augite is mainly 

 ju-esent in the form of granules, which, with felspar in a similar con- 

 dition, forms the ground-mass of tVie rock. A large number of clear 

 isotropic octagonal or rounded masses are to be attributed to the 



