370 PEELITIC CRACKS IN QUARTZ. [Aug. 1 894. 



microlites. Faulting in flow-structure of the third type is seen in 

 fig. 1, p. 371. 



Occasionally perlitic cracks may be seen to traverse the trichites 

 (as in the slide from which fig. 1 is taken), and at other times a 

 similar fissure may be seen passing through a felspar-microlite. 

 These phenomena are, however, difficult to observe, and are not likely 

 to be common, because the perlitic cracks circle round the edge of 

 the phenocrysts and the flow-structure determines the microlites to 

 take a similar direction. It would be impossible to tell whether a 

 crack, usually more or less oblique to the section, cut through the 

 breadth of such minute microlites. The depolarizing granules are 

 similarly seen to be cut through, without displacement, by the 

 cracks. 



A curious feature of the matrix-perlites is the frequent bleaching 

 of their interior. The colourless centres are not as a rule devitrified, 

 although this may sometimes occur ; generally they are made 

 up simply of a colourless glass, which passes gradually into the 

 normal brown glass, and then, very occasionally, in the centre there 

 is a group of fairly developed felspar-microlites. Inside a few of 

 the colourless perlites the glass has been broken up by a number of 

 irregular star-like fissures. In extremely few instances the very 

 centre of the perlite is darker than the rest, and then there is a 

 slight aggregation of trichites at that spot. 



There is no optical evidence of strain in the perlites. "Where the 

 flow-structure is pronounced and of type 2 the perlites may occur 

 only, or more frequently and with greater perfection, in the glass ; 

 or they may cut both the glass and the lithoidal portion ; or, in still 

 rarer cases, a single perlite will involve both structures, but its cracks 

 will not be so clearly visible where they cross the lithoidal band. 



IV. Perlitic Structure in Quartz. 



In many of the rounded porphyritic crystals of quartz in this 

 rock very fair examples of perlitic structure are to be seen, which are 

 at least as perfect as those produced by the rapid cooling of Canada 

 balsam. Indeed, I am able to show that not only are these cracks 

 of a general perlitic nature, but that (1) the cracks pass outwards 

 from crystals to matrix ; (2) the matrix-perlites are sometimes com- 

 pleted in the quartz ; (3) perlites are formed in quartz and completed 

 in the matrix outside ; (4) the polygonal cracks sometimes extend 

 into the quartz and have the same characters as in the matrix ; 

 (5) the radial cracks are also to be found in the quartz, and even 

 enter the quartz from the matrix. 



The general nature of the perlitic cracks in the quartz can be 

 judged by reference to the figures. The most characteristic is, 

 perhaps, that shown in fig. 2, where the inner crack is very perfect, 

 and two outer cracks are connected with it by radial fissures. The 

 same characters are seen in PI. XVIII. fig. 4, in which the whole of 

 the quartz is traversed by a series of curved cracks of a general 



