452 S. ALLPORT ON PITCHSTONES AND PERLITES FEOM 



filtration of foreign matter in solution ; and that they were caused by 

 the strain produced during contraction of the brittle glass is rendered 

 evident by such examples as the following, in which may be traced 

 nearly every gradation between long rectilinear fissures or joints and 

 those forming the typical perlitic structure. Fig. 1 (PI. XX.) repre- 

 sents the arrangement observed in a section of perlitic pitchstone from 

 near Meissen. Several roughly parallel joints divide the glass into 

 small columns ; and these are again traversed by cross joints, which 

 thus form minute rectangular blocks. In the central one (a) a 

 spheroid is formed by four curved lines, which clearly branch off 

 from the lateral joints, and round off the corners of the rectangle. 

 In the upper compartment there are three distinct spheroids ; and in 

 the lower one there are two of irregular shape. Fig. 2 is from 

 another part of the same slice. In fig. 3 the formation differs in this 

 respect, that in both columns several spheroids are piled one on the 

 other without any intervening cross joints. In a typical perlite from 

 Cabo de Gata in Granada, there are numerous large spheroids, in which 

 many of smaller dimensions are enclosed (see fig. 4). In nearly all 

 the specimens examined the glass is divided into areas of various 

 sizes and forms by a number of parallel or diverging straight lines 

 (joints), the intervening spaces being frequently crowded with sphe- 

 roids, many of which are flattened against each other (see fig. 5, 

 PI. XX.). 



An examination of all the facts leads to the conclusion that the 

 perlitic texture is purely a phenomenon of contraction ; and I quite 

 agree with Mr. Rutley * that the explanation of the spheroidal 

 structure in basalt recently laid before the Society by the Eev. T. G. 

 Bonney t is a closely parallel case. There is, however, this differ- 

 ence — that, in the case of basalt, the comparatively tough tex- 

 ture produced by the interlacing of its crystalline constituents would 

 enable it to resist the actual fracture so frequently exhibited by the 

 more brittle perlites. 



The perlites and other vitreous rocks usually contain numerous 

 minute microliths, which have been described by Zirkel under the 

 names of belonites and trichites ; their mode of occurrence and re- 

 lation to the superinduced spheroidal (perlitic) structure deserve 

 special attention. 



The belonites are minute translucent prisms, either colourless or 

 of pale yellow or greenish shades ; they occur in immense numbers, 

 and are frequently crowded together in stream-like bands, with their 

 long axes lying in one general direction. Whenever a stream 

 encounters any small crystal of felspar, quartz, or mica imbedded in 

 the mass, the belonites are invariably diverted from their course and 

 bend round it, their axes lying parallel with, its sides. Their 

 relation to the perlitic spheroids, however, is totally different ; 

 for instead of winding round them, they continue their course 

 uninterruptedly through and across them (see fig. 8). It becomes 



* ' ' On some Structures in Obsidian," &c, read before the Royal Microscopical 

 Society, March 1876. 



t Quart. Journ. Geol. Soc. yol. xxxii. p. 140. 



