430 Scientific Proceedings, Royal Dublin Society. 



shows a perfect external form is seen under crossed nicols to be 

 partly fringed by a secondary growth, possessing an irregular 

 and ragged outline. In ordinary light there is, however, no trace 

 of a development of this nature ; and the contours of the crystal 

 stand out in sharp relief from the surrounding matrix. It is 

 only under crossed nicols that the outgrowth becomes visible. 

 "We are then able to observe that the material composing this 

 fringe is largely felspar, with minute fibres of intercalated quartz 

 — in fact, its constitution is identical with that of the spherulites. 

 The felspathic substance extinguishes coincidently with the felspar 

 to which it is attached. As yet I have only observed the growth 

 upon the faces P and x. But one of these faces sometimes shows 

 an attachment of pseudo-spherulites, whilst the other will then 

 present the outgrowth described above ; so we have the two struc- 

 tures developed side by side upon one and the same crystal. It is 

 a very significant circumstance that such crystals have only under- 

 gone enlargement where they would be in contact with a glassy 

 matrix, and that where other crystals lie against them all growth 

 upon the contact surfaces is suspended. I will refer to this inte- 

 resting subject later on. Some serpentinous masses point to the 

 original presence of a non-aluminous silicate. Epidote is very 

 common ; but porphyritic quartz is not apparent. There is a goodly 

 quantity of quartz in the groundmass, but it seems to be for the 

 most part of a secondary nature. The groundmass of the rock is 

 distinctly micro-crystalline ; but this aspect may possibly be due 

 to some extent to the infiltration of secondary silica. 



It may not unreasonably be asked whether we are not dealing 

 with glassy rocks which have undergone devitrification. The 

 question would be a natural one, seeing that many felsites and 

 felsite-porphyries are now judged to be nothing else than devitri- 

 fied obsidians, pitchstones, and vitrophyres; but the subject pre- 

 sents numerous and serious difficulties. Our present knowledge 

 does not permit us to distinguish these from those rocks in which 

 the double refraction of the felsitic matter is a consequence of 

 primary devitrification, or that which is the consequence of actions 

 accompanying solidification. 1 In micro-granites and granophyres 

 the double refraction of the matrix, due to the fact that the 



1 Teall, Op. cit., p. 293. 



