"Vol. 50.] OF PERLITIC CRACKS IN QUARTZ. 373 



fig. 5 a large matrix-perlite is seen bordering a second, which has 

 a perfect set of radial fissures. The outer crack of the first perlite, 

 which goes off at a tangent, enters the quartz-crystal in the lower 

 part of the figure, traverses it for a short distance, and then passes 

 out into the matrix. The angle made by this crack with the average 

 circumference of the perlite is not greater than that of its inner 

 cracks where traversing the matrix only. A couple of radial 

 fissures are given off by it, and, further, there is a set of polygonal 

 cracks surrounding the principal perlite, with branch cracks sur- 

 rounding subordinate ones ; of these, two major cracks and one of a 

 minor character occur and join up in the quartz. PI. XVIII. fig. 6 

 gives an excellent example of a typical matrix-perlite contained in 

 polygonal cracks, and giving off a great number of radial fissures. 

 Some of the polygonal cracks are confined to the matrix, others just 

 traverse the quartz, and radial cracks passing from the perlitic 

 cracks of the matrix are seen to pass into the quartz and stop there 

 at the junction with the polygonal crack (see PI. XVIII. fig. 6 a). 



When traversing quartz the cracks are generally filled with a 

 deposit, sometimes of magnetite, generally associated with small 

 granules or plates, probably of specular iron, and usually also 

 with a deposit of a colourless mineral, which is likely to be quartz. 

 A curious reticulated structure is seen in the latter, with which 

 also a little chlorite occurs. 



I have reserved till last the occurrence of the cracks in slide I 861, 

 cut from the lithoidal variety referred to on p. 368. The matrix 

 is traversed only by polygonal cracks, amongst which it is very rare 

 to find anything approaching a perlite; indeed, I have only seen 

 one in the slide. The matrix is not glassy, but consists of recog- 

 nizable microlites of felspar (varying from '001 to "004 in. in 

 length), often forked, and set in a cryptocrystalline granular sub- 

 stance. Immediately round the quartz, perlitic cracks are very 

 frequent, but they very rarely avoid the quartz altogether ; the 

 large polygonal cracks of the matrix, however, in most cases avoid 

 the quartz, passing round the crystals or being deflected very con- 

 siderably before entering them. On reaching the felspars these 

 polygonal cracks generally pass obliquely through them, stepping 

 down along the cleavage pianes. In the matrix cracks may some- 

 times be seen, though rarely for the reason given above, to cut 

 and traverse felspar-microlites — making it quite certain that the 

 lithoidal structure existed before the perlitic cracking, and that the 

 matrix is in all probability original and not a devitrified glass. 



V. Conclusions. 



The facts recorded above allow us to draw the following deduc- 

 tions : — 



1. That perlitic cracking is not inconsistent with crystalline 

 structure, but is only likely to be developed where there is no good 

 cleavage along which the strain would be more easily relieved. 



