110 J. H. Teall — Checiot Quartz -FelsUes and Augite- Granites. 



pushed them a little further back. Let us endeavour to take stock 

 of our present position in relation to this subject. 



In discussing the question it is necessary to remember that 

 the microscope does not enable us to approach the limits of the 

 molecular world. There is room for a universe of phenomena, so to 

 speak, between the limit of microscopic visibilit}'^ and that of mole- 

 cular structure. Hence, many structures and modes of association 

 in minerals which we observe macroscopically and microscopically, 

 as well as others which we cannot observe, may exist in this ultra- 

 microscopic universe. Now many rocks which show a crj'pto- 

 crystalline sti'ucture, when examined in thick sections and with low 

 powers, may be resolved, by I'educing the thickness of the sections 

 and by using higher powers, either into a micro-crystalline aggregate 

 or into doubly refracting particles, and a glassy or micro-felsitic 

 base. If the doubly refracting particles are small in comparison 

 with the thickness of the slide, then, owing to their superposition, 

 they will give rise to the indefinite aspect under crossed Nicols 

 which is the special characteristic of crypto-crystalline matter. 



These considerations lead us, therefore, to the conclusion that the 

 term crypto-crystalline includes different things and is useful merely 

 for the purpose of expressing our ignorance as to the precise con- 

 dition of the matter to which it is applied. It is an expression which 

 is rendered necessary in consequence of imperfection in our methods 

 of observation, and does not correspond with anything definite in the 

 nature of things. It is a subjective and not an objective tei'm, and 

 one that we must therefore use for the purpose of expressing our 

 ignorance, and not for the purpose of concealing it. In concluding 

 this discussion it is well to remark that the felsophyres, granophyres, 

 and micro-granites cannot be distinguished from each other by 

 macroscopic examination. All that we can do in the field is to 

 separate the quartz-porphyries with a vitreous from those with a 

 felsitic ground-mass. The former may be termed vitrophyres, while 

 for the latter our own term, quartz-felsite, is veiy appropriate. 



If we consider the porphyry ground-mass from the Eetiological 

 point of view, then we have to recognize the fact that the micro- and 

 crypto-crystalline ground-masses may owe their double refraction 

 either to crystallization at the time of consolidation or to the sub- 

 sequent devitrification of an original glassy substance. Perlitio 

 structure, so far as we know, can only arise during the consolidation 

 of a glass, ancl we may therefore fairly conclude with Mr. Allport ^ 

 and others that rocks which show this structure were originally in the 

 glassy condition. In many cases, however, a perlitic rock will split 

 up under crossed Nicols into a micro-crystalline aggregate. So far 

 as I know, we have at present, in the absence of perlitic or some 

 other structure equally characteristic of a glass, no test by which 

 we can distinguish an original micro- or crypto-crystalline ground- 

 mass from one which has been produced by subsequent devitrifica- 

 tion. • Here again we have to make a confession of ignorance. 



If we look at the quartz-porphyries in their relations to other 



■i On Devitrified Pitchstones and Perlites of Shropshire, Q.J.G.S. vol. xxxiii. p. 449. 



