﻿part 3] THE PSEUDOTACHTLTTE OF PARIJS. 205 



Fig. 13. — PseudotachyJyte 200 yards above the boathouse. 



courses (text-figures 2, 3, 9-13) and their direction is entirely independent 

 of the foliation of the granite (text-figures 3, 5, 6, 8, 10). 



They thicken and thin repeatedly and rapidly (text-figures 6, 7, 9, 10-12), 

 give off branches at high and low angles (text-figures 2-4, 7, 8, 10-12), and 

 often anastomose in the most complicated way (text-figures 10-12). 



In innumerable cases they thin out and terminate blindly (text-figures 1-3, 

 7, 8, & PI. XVII, fig. 1). 



The contact with the surrounding granite, as seen with the naked eye, is 

 perfectly abrupt. The granite is never sheared parallel to the course of the 

 veins, its texture remains unchanged, and the felspars continue to show 

 large bright cleavage-faces and straight twinning lamellae, right up to the 

 surface of contact (PI. XIX. fig. 1). 



The junction-line is often nearly straight for distances of many feet, 

 especially in the case of those veins which are not more than a few inches 

 thick (text-figures 1, 2, 4, 5, & PI. XVII, fig. 1) ; in other cases it is strongly 

 serrated (PI. XVI). 



The veins vary in width from a fraction of an inch up to 2 or 3 feet ; 

 but in the thicker veins there are always numerous inclusions or floaters of 

 granite which occupy a large proportion of the stated width, and the black 

 base is often reduced to the role of a mere cement for the floaters (PI. XVI, & 

 PI. XVII, fig. 2). The floaters range from boulders a couple of feet in diameter 

 down to minute grains, and they are far more frequently rounded or turbinate 

 than angular (PI. XVII). In an average sample about 20 or 25 per cent., 

 but in some cases (PI. XVII, fig. 2) as much as 80 per cent., of the contents 

 of a vein consists of boulders and fragments visible to the naked eye. 

 • The black base in which the granite floaters are embedded is a compact 

 black rock, like a microcrystalline basalt or tichylyte. capable of taking on a 

 high polish. Apart from grains of quartz and felspar derived from the granite. 



