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removed all difficulties. The examination of thin slices has enabled us to 

 recognize that in many cases the felsitic matrix of the quartz-porphyries is 

 a definite crystalline aggregate. VOGELSANG, as we have already seen, 

 proposed the term granophyre for such rocks. ROSENBUSCH proved that 

 the granophyres of VOGELSANG could be separated into two more or less 

 distinct groups : (1) those in which the constituent minerals (quartz and 

 felspar) occur as individual grains or crystals and (2) those in which the 

 two minerals are inter-crystallized according to more or less definite laws. 

 The former he proposed to call micro-granites, the latter granophyres. 

 When, however, we have separated the micro-granites and the granophyres 

 there yet remain many rocks in which the ground -mass is not composed of 

 glass. What is the nature of this ground-mass ? In many cases it 

 gives under crossed nicols the vague kind of aggregate polarization 

 which is generally known as crypto-crystalline. Now the term 

 crypto-crystalline merely expresses the fact that the substance is 

 composed either wholly or in part of double-refracting particles which 

 are so small as not to be capable of definite recognition by the 

 methods employed. Matter which appears crypto-crystalline when 

 examined in thick sections and with low powers may often be resolved 

 into a micro-crystalline aggregate or into an aggregate of double-refracting 

 particles and a glassy base when examined in thinner sections and with 

 higher powers. In dealing with this crypto-crystalline ground-mass we are 

 brought face to face with difficulties of the same kind as those experienced 

 by the older petrographers in dealing with the felsitic base. The 

 introduction of the microscope has not removed the difficulties : it has 

 merely pushed them further back. 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 condition of the matter to which it is applied. It is an expression 

 which is rendered necessary in consequence of imperfection in our methods 

 of obssrvation, and d)3s not correspond with anything definite in the 

 nature of things. It is a subjective and not an objective term, and one we 

 must therefore use for the purpose of expressing our ignorance rather than 

 for the purpose of concealing it. 



Seeing that the term felsite is applied to matter which is incapable of 

 resolution by examination with the naked eye or with a simple lens, it 

 would seem perfectly natural that the term micro-felsite should be applied 

 to the matter which is incapable of resolution under the microscope. This 

 is approximately the sense in which it was used by ZIRKEL and VOGELSANG. 

 ROSENBUSCH has, however, strongly argued against this use of the term. It 

 is, however, somewhat difficult to understand precisely the view of the last- 

 mentioned author. In the last edition (1887) of his work on the massive 

 rocks, after referring to micro-crystalline, crypto-crystalline and glassy 

 matter, he describes "a thoroughly isotropic substance, sometimes colourless, 

 sometimes greyish, often, however, yellowish or brownish, which may be 

 distinguished from a glass with devitrification products by the fact that it is 

 not structureless, but is composed entirely, or almost entirely, of extremely 



