﻿part 3] 



THE PSEUDOT ACH YL VTE OF PAItUS. 



221 



between fused material and mylonite ought to be represented. 

 There did not appear to be such a gradation ; while, on the other 

 hand, there was abundant evidence that the material had possessed 

 a sufficiently high temperature to fuse the minerals of the invaded 

 rocks. The question then arose : why had the black rock crystal- 

 lized so badly ? One explanation might lie in the assumption that 

 the intrusion was feebly supplied with volatile fluxes ; and such a 

 condition would naturally arise if some deep-seated rock that had 

 previously crystallized and lost its fluxes, were again to be raised 

 to its melting-point — by severe crushing, or rise of the geotherms, 

 or any other cause. 



The Author stated, in the abstract of his paper, that the compo- 

 sition of the rock was practically that of a granodiorite, and that it 

 might correspond to an average of the granite^pes in which the 

 veins occurred. The speaker said that he could not agree with 

 either of these statements, for the analysis showed about 10 per 

 cent, of alkalies, with soda nearly twice as abundant as potash. The 

 speaker suggested that, if the Author could have specimens of the 

 pseudotachylyte and of the red and grey granites sent to England 

 for analysis, the results could not fail to throw more light on the 

 origin of this most puzzling of rocks. 



Dr. J. W. Evans thought that it was clear, in the majority of 

 cases, that the material of pseudotachylyte had been injected into 

 the rock in a fluid or pasty condition, as the result of pressure at 

 right angles to the general direction of injection. It did not 

 appear to be derived from an extraneous magma, but at least in 

 some cases, including that described by the Author, the matrix had 

 been in a molten state. It was doubtful whether the heat pro- 

 duced by earth-movements was ever sufficient to cause an appreciable 

 rise in temperature, and the speaker was inclined to attribute the 

 melting to a regional rise in temperature which did not melt the 

 granite because this had lost its magmatic water. The intrusive 

 material may have been formed from a basic segregation, more 

 easily fusible on account of the larger proportion of iron and 

 alkalies. Such segregations would in many cases determine planes 

 of weakness in the rock. The brecciation of the granitic material 

 included in the veins might, in some cases, be attributed to the 

 forcible injection of the latter. 



