184 ME. G. A. J, COLE ON THE ALTEEATION OF 



a central vesicle, produced by the expansion of gases while the lava 

 was still cooling down (PL IX. fig. 1). 



An experiment as to the degree of solubility in acid of the spheru- 

 lite and the matrix respectively yielded only an unsatisfactory 

 result. Equal quantities of the powdered substances gave, after 

 eight days' immersion, with two boilings, in strong hydrochloric 

 acid, the following percentages of material dissolved : — 



Gf-lassy matrix. Spherulite. 



7-57 8-80 



Although the portions of the spherulite were chosen with great 

 care, some of the above difference might be due to minute included 

 veins of calcite. On the other hand it is possible that even such a 

 slight variation in behaviour as that indicated above might be 

 greatly intensified by the temperature, and perhaps pressure, at 

 which the natural solvents worked. 



Angular secondary aggregates, apparently very similar to those of 

 Zwickau, occur in other Saxon porphyries and pitchstones, and are 

 recorded, as one might expect, from the most highly altered zones*. 

 These igneous sheets are associated with Rothliegende sandstones 

 and conglomerates, which are sometimes cemented by silica, some- 

 times by carbonate of lime, and which contain occasional calcareous 

 layers ; when, therefore, the felsitic spherules had once been hol- 

 lowed out (a process begun, in all likelihood, while the volcanic 

 centre was still active), the calcite and chalcedony may easily have 

 infiltered from the surrounding sedimentary rocks. 



The process of secondary devitrificationf tends often to lessen 

 still further the difference between the spherulites and the matrix ; 

 so that we may at any time be confronted by an altered lava con- 

 taining angular shreds of quartz, chalcedony, &c., which, had we 

 none of the intervening stages, might even be regarded as fragments 

 picked up during flow. 



The red rocks of the Wrockwardine (Wrekin) area have preserved 

 their original structures well ; but among them is a coarsely sphe- 

 rulitic layer in which the formation of secondary quartz-masses has 

 in places gone to an extreme. As in the historic Zwickau pitch- 

 stone and many later examples, the banding often traverses the 

 larger spherules and carries trains of small ones through them. The 

 largest spherulites are sometimes aggregates of those of medium 

 size, and become, indeed, in this way elongate, knotty, or irregular. 

 The development of quartz in their interior has been previously de- 

 scribed t ; but I confess that on recently revisiting the area, I was 

 unprepared for the scale on which this alteration has taken place. 

 One spheroidal mass, for example, measures 11 centim. (4| inches) 

 in diameter, and yet the residual red material is often not more 



* Lehmann and Siegert. Erlauterungen zur geol. Karte des Konigr. Sachseii, 

 Section Hohenstein, pp. 44, 45, & 53. 



t See Bonney, Proc. Geol. Soc. 1885, p. 87. 



I Q. J. Gr. S. vol. xli. p. 167. Cf. Monteiro, " Description mineralogique du 

 Pyromeride globaire de Corse." Journal des Mines, tome xxxv. (1814), p. 351. 



