THE LOWER-SILURIAN DISTRICT OF SHROPSHIRE. 457 



the object alternately to right and left, the previously well-defined 

 granules gradually assume shadowy indefinite forms of variable 

 dimensions, alternately appearing and disappearing ; while the per- 

 litic curved lines are seen to pass continuously through them, and 

 even the minute belonites and trichites appear with opposite ends 

 enclosed in two of the adjacent pseudo-granules. 



When the devitrification appears to be complete, and a granular 

 mosaic structure the most distinct, there is not the slightest appear- 

 ance of any such fragmental character when examined in ordinary 

 light. Under all powers of the microscope the matrix then seems 

 to be a perfect glass traversed by straight or tortuous streams of 

 microliths and intersected by the perlitic fissures already described. 



Although these investigations show conclusively that certain 

 molecular changes have taken place, it should not be overlooked 

 that the structure resulting from devitrification differs in character 

 from that of a true felsite ; and I think the rocks here described 

 afford no evidence in favour of the view held by Vogelsang and 

 others, that the base of the so-called quartz-porphyries may have 

 been originally of a glassy nature. 



The thin sections are frequently traversed by narrow veins, which 

 pass through all the constituents, as shown in fig. 10. A fine 

 fissure has here cut through the matrix, a spherulite, and an 

 enclosed crystal of felspar. In all cases these veins appear to con- 

 sist of the same substance as the devitrified matrix ; and when the 

 latter is penetrated b} r one of them, it is impossible to distinguish 

 the one from the other except in certain positions of the prisms, 

 when the continuity of the vein may still be detected. 



Quartz- veins are not uncommon ; and in some parts of the sphe- 

 rulitic rock chalcedonic nodules are rather abundant. 



Although the microscopical structure of these rocks is in itself deci- 

 sive as to their origin, I may add that their chemical composition 

 is also in perfect accordance with their other characters. Mr. J. A. 

 Phillips kindly made for me an analysis in duplicate of a specimen 

 of the devitrified perlitic pitchstone, with the results shown in the 

 following Table, in which I have also included analyses of two 

 Miocene perlites : — 



Spec. grav. 2-62. I. II. HI. IV. 



Water 1-47 1*46 3-50 2-90 



Silica 72-18 72-19 72-52 73-00 



Alumina 14-46 14-44 13-72 12-31 



Ferric oxide 1-78 1-59 1 9 -, 9 n - 



Ferrous oxide 0-91 0-91 f * Kl J ' U ° 



Oxide of manganese . . trace. trace. 



Lime 0-92 093 1*15 1-20 



Magnesia trace. trace. 0*45 1-47 



Potash 6-10 6-14 5'68 5-96 



Soda 1-92 1-96 1-15 1-36 



99-74 99-62 100-25 100-25 



2h2 



