96 GEOLOGY OF THE SON VALLEY, ETC. 



found side by side with them ; and that some rhyolite fragments contain 

 idiomorphic felspars with subsequent enlargement of the felspar 

 which could only have taken place when the mass was partly molten, 

 and which shows that the crystalline nature of the groundmass is an 

 original feature at least in the immediate neighbourhood of the por- 

 phyritic crystals. But there is no doubt as to the large extent to 

 which secondary crystallisation of quartz has taken place, so that 

 angular fragments of micro-crystalline rhyolite, whose shapes are per- 

 fectly definite in ordinary light, appear with quite confused bound- 

 aries in polarised light, and can scarcely be distinguished from the sur- 

 rounding aggregate of minute grains. These angular fragments ap- 

 parently quite similar to the matrix containing them may be observed 

 even in hand-specimens of the porcellanites. This is easily understood 

 on the supposition that the rocks are the result of showers of volcanic 

 dust in which the fragments lack the uniformity in size obtained in 

 a purely sedimentary rock deposited from water flowing with a 

 definite speed, and the extent to which secondary crystallisation has 

 gone on gives great similarity to both the matrix and the included 

 fragments. In fact under the microscope, the " porcellanites " appear 

 quite similar to the matrix of the " trappoids. " The two rocks are 

 practically identical, the only difference being that the porcel- 

 lanites do not contain the larger fragments 

 characteristic of the " trappoids. " The perfectly 

 homogeneous appearance and compact nature of the rock is due, as 

 above stated, to secondary crystallisation of quartz, but in a thin 

 section the component particles when not actually visible with the 

 naked eye, may be discerned at once by the aid of a pocket lens. 

 The sections are resolved by the lowest powers of the microscope, 

 the boundaries of the crystalline components being particularly clear 

 in polarised light. The size of these particles varies between 

 extremely wide limits, just as in the case of the trappoids, and 

 without any regular arrangement of grains of one particular size in 

 definite layers. The minerals are exactly the same as in the case 

 ( 96 ) 



