Vol. 50.] OF PERUTIC CRACKS IN QUARTZ. 369 



glass, and sometimes these are surrounded by a few small felspar- 

 crystals accompanied by magnetite. 



There is nothing unusual in the development of the perlitic 

 structure in the glass of these rocks. It is first traversed by two 

 sets of polygonal cracks, running very rudely at right angles to 

 one another, of which one set is generally much better developed 

 than the other. The cracks cross where they meet, and usually one 

 gives off a crack which curves rouud and joins the other tangen- 

 tially (PI. XVIII. fig. 2) ; inside the spaces thus formed come the 

 perlites, which vary from "05 in. to - 005 in. in diameter. Some- 

 times they are simple, with circumferential cracks passing down to 

 a very small scale ; but very frequently one perlite contains several 

 others, like those figured by Allport 1 and the compound spheroids 

 figured by Bonney. 2 Occasionally two perlites have one common 

 flat side, and, in one case that I observed, two perlites have impressed 

 a third which lies between them. 



The cracks are usually filled up with a crystalline substance, 

 which depolarizes under crossed nicols and gives a maximum extinc- 

 tion in that part of the crack to which the short axis of one of the 

 nicols is tangential. This corresponds with several instances 

 described by Rutley and with the infilling of perlitic cracks in the 

 pitchstone (' felsit-pechstein ') of Buschbad, near Meissen. In 

 consequence of this infilling the cracks are always best, and often 

 only, visible with crossed nicols ; indeed, prolonged observation with 

 high and low powers is necessary to appreciate the full perfection 

 of the contraction-structures in the rock. Generally there is a dust 

 of magnetite seen in those cracks which are oblique to the surface 

 of the section, and this, in one section, increases in quantity to such 

 an extent as to make the whole of the cracks black and opaque. 



A very common feature is a close -set series of radial cracks which 

 run at right angles to the concentric cracks. These are very well 

 seen in PL XVIII. figs. 1, 2, and 6, and though they are more usual 

 across the outer cracks of the perlite, they are by no means absent 

 from the inner and innermost cracks. They are not usually con- 

 tinuous for any considerable distance, but are replaced by others 

 on different lines ; and this is especially the case in opposite sides of 

 a perlitic crack. Similar fissures occur at right angles to the 

 polygonal cracks, as shown in PI. XVIII. figs. 2 and 3. 



The contraction-cracks occasionally cause faulting in the flow- 

 structure. This latter structure is marked in several ways : — (1) by 

 light and dark glassy bands ; (2) by darker glassy bands alternating 

 with lighter bands, which have a larger number of felspar-microlites, 

 or in which the groundmass is somewhat devitrified ; (3) by dark, 

 often twisted bands, in which there is an abundance of excessively 

 minute magnetite-dust ; (4) by stream-lines of dark trichites and 

 the general arrangement of them ; (5) by streams of felspar- 



1 Quart. Journ. Geol. Soc. vol. xxxiii. (1877) pi. xx. figs. 3 and 4. 



2 Op. cit. vol. xxxii. (1876) p. 151, fig. 13. 



