﻿part 3] 



THE PSEUDOTACHYLYTE OF PARIJS. 



219 



from the fact that biotite is the first mineral to fuse. In preparing 

 the powder of 2 a for analysis, the larger fragments of quartz and 

 felspar were probably rejected (at least, that was my own procedure) ; 

 but no biotite was rejected with them, hence the enrichment of 

 the base in the constituents of biotite. 



All three analyses fall, in the quantitative classification, into 

 lassenose (I, 4,2,4) — a group containing many plagioclase-granites 

 and dacites. 



The conclusion to which my re-examination of the rocks, coupled 

 with Mr. Page's analysis, had already led me is, therefore, com- 

 pletely confirmed b}*- Dr. Harwood's work. The pseudotachy- 

 lyte has originated from the granite itself through 

 melting, caused (as I have shown) not by shearing but 

 by shock, or, alternatively, by gas-fluxing. 



I wish to offer my warmest thanks to Dr. Harwood for his 

 kindness in performing the above analyses, and thus enabling the 

 investigation of these remarkable rocks to be completed ; and also 

 to Dr. Holmes for so kindly interesting himself in the matter. 



EXPLANATION OP PLATES XVI-XIX. 

 Plate XVI. 



A large vein of pseudotachylyte exposed in a cutting - made for one of the 

 piers of the new bridge below Parijs (Orange Free State). 



Plate XVII. 



Fig. 1. A vein coming up vertically, turning over to a nearly horizontal plane, 

 and thinning out. Thickness of the vein = about 6 inches in the 

 middle of the photograph. Exposed in a cutting at the bridge. 

 (The detail of the blind end of this vein is shown in text-figure 8, 

 p. 203.) 



2. A boulder from one of the large veins, thrown out during blasting 

 operations at the weir, above Parijs : shows rounded floaters of 

 granite and gneiss. A foot-rule has been placed alongside, in order 

 to give the scale. 



Plate XVIII. 



Fig. 1. Melted felspar at the margin of a vein. The clear white areas are 

 quartz-grains. X 15 diameters. (See p. 207.) 

 2. Section near the margin of a vein : it shows granite fragments being 

 detached, apparently corroded, and floated away. X 15 diameters. 

 (See p. 207.) 



Plate XIX. 



Fig. 1 . Margin of a vein, showing a plagioclase-crystal which has been broken 

 but not sheared. Crossed nicols. X 15 diameters. (See p. 207.) 



2. Melted felspar at the margin of a vein. The clear colourless areas 



are quartz-grains. X 15 diameters. (See p. 207.) 



3. Pseudotachylyte with ground-mass of type 2, full of minute hornblende- 



prisms, x 45 diameters. (See p. 206.) 



