318 



MESSES. G. A. J. COLE AND J. ^Y. GREGORY 



proportions. Microscopic sections reveal a number of transitional 

 forms, with delicate radial groups of felspar and brown interstitial 

 glass, such as have been referred to in our description of the diabase- 

 lavas. We have also found fragments of the porphjTitic andesite or 

 diabase of the arete of Le Chenaillet — a fact that confirms the 

 practical contemporaneity of the great dykes and these agglomerates. 

 Of course, on the other hand, these fragments would by themselves 

 support the view that the agglomerates are friction-breccias. The 

 matrix of the tuff has generall}^ been extensively altered, and a zone 

 of epidote and other clear secondary minerals, accompanied by opaque 

 granules, has often fonned around the included fragments. Par- 

 ticles of basic glass, rich in globulites and minute stellar aggregates, 

 are freely scattered ; and many even of the stony-looking and 

 darker fragments prove, between crossed nicols, to retain their 

 original amorphous character (PI. XIII. fig. 4). 



Pig. 6. — West SJoj^e of Le Chenaillet, sliowing €11)1:6-111:6 forms 

 of the Variolite Tuff. 



Le Chenaillet. 



Gabbro of the 

 Col du Goiidraii. 



The secondary mineralization of the matrix has given rise to a 

 handsome rock on the west of Mt. La Plane, where dark-green 

 particles of variolite lie in a ground of bright yellow epidote. 

 Where, moreover, the tuffs come in contact with the serpentine, 

 hydrous magnesian silicates have permeated the mass, and even the 

 variolitic fragments have become softened and decomposed. The 

 resulting serpentinous tuff is, on J^lt. La Plane and at the Col du 

 Chenaillet, difficult to distinguish from the breccia of the serpentine 

 itself; but we have found no passage between these rocks, the 

 variolite never occurring in the breccia, and the wisps of diallage or 



