﻿part 3] 



THE PSEUDOTACHYLTTE OE PAHIJS. 



207 



others. It does not seem to constitute entire veins, but to occur in patches 

 within rocks which also show type 2. Considering the invariable blackness, 

 of the rocks, it is surprising to find that the section often shows very little of 

 dark or opaque minerals, but consists either of a honeycomb of polygonal 

 spherulites of a dark-brown colour, showing a perfect cross between crossed 

 nicols (PI. XIX, fig. 4), or of a felt of felspar-microlites with a subordinate 

 amount of magnetite-dust and a few green scales in the interstices (PI. XIX, 

 fig. 5). The microlites rise to 0 - 05 mm. in length and O'Ol in width. They 

 are all twinned, and, although their small size makes the determination diffi- 

 cult, it seems that both Carlsbad and albite twins are present. The extinction 

 is always straight or practically so, and it is probable that both orthoclase and 

 oligoclase are represented. The arrangement of the microlites is generally 

 radial, not parallel, and it is thought that the stellate groups of microlites. 

 have developed out of spherulites. 



The granite- walls and inclusions. — I have cut various, 

 sections across the junction between granite and pseudotachylyte, 

 and I find that in general the granite shows no change which can 

 be traced farther than 1 or 2 mm. from the edge. In some cases, 

 this junction is absolutely sharp, and the granite betrays no change 

 whatever. Such a junction is shown in PI. XIX, fig. 1, where a 

 single plagioclase crystal is seen to be cut sharply across, as if by 

 a knife ; a few tiny cracks are observed at right angles to the- 

 fractured surface, and along these there has been a very slight, 

 faulting of parts of the crystal ; but, with this exception, the twin- 

 ning lamellae are perfectly straight, showing that there has been no 

 shearing of the granite parallel to the vein. This is not an isolated 

 instance — I have observed the same feature in several slides*. The 

 general conclusion as to the absence of shearing in the granite 

 holds rigidly for every case that I have studied in the field or in 

 the laboratory. In one case a crystal of titanite was found, half of 

 which is embedded in the granite- wall, while the other half projects 

 well into the tachylyte-vein ; here is proof positive of the absence 

 of shearing. 



In other cases the quartz- and felspar-crystals of the granite, 

 within a space of 1 or 2 millimetres from the edge, are consider- 

 ably cracked, show a very irregular extinction, and are invaded 

 by tongues of pseudotachylyte. Fragments of quartz and felspar 

 actually enclosed in the vein-rock are much cracked, the quartz- 

 grains in particular showing a beautifully minute mosaic structure 

 between crossed nicols. This granulation is unaccompanied by 

 schistosity, and is such as might have been caused by rapid heating 

 of the crystals. PI. XVIII, fig. 2 shows granitic fragments floating 

 in tachylyte just at the margin of a vein. The shadowy grains are 

 felspar, the clear white grains are quartz. All the grains show 

 fretted, embayed outlines, and the suggestion of active corrosion by 

 the black base is very strong. Actual proof of melting of felspar is 

 contained in PI. XVIII, fig. 1 & PL XIX, fig. 2, which are taken 

 from the margin of a large vein. In these photographs the quartz- 

 grains stand out in clear relief from a matrix of half -melted 

 felspar which shows viscous flow-structure. 



Sometimes the included felspars are red, and present a compact, 



