132 



HARDWICKE'S SCIENCE-GOSSIP. 



parts one sees little rounded concretions formed of 

 radiating crystals, and called spherulites. Now the 

 combination of all these characters leads in a 

 remarkable manner to the unravelling of the rock's 

 history. 



When we come to examine thin sections of these 

 felsites under the microscope, the origin and relative 

 importance of these several structures can be made 

 out. The porphyritic crystals of felspar are set in 

 a matrix of crypto-crystalline material ; that is, a 

 matrix which gives on the whole the reactions of a 

 crystalline body, but in which the distinct crystals 

 are not well defined. The flow-lines are resolved 

 into irregular lines of microliths, tiny embryo 

 crystals, whose long axes maintain a general direction 

 along the lines of flow. 



The cracks of the perlites are beautifully shown, 



consolidation appears to have been the same. These 

 structures therefore tell us that the old Pebidian 

 felsites were once glassy pitchstones or trachytes, and 

 that they cooled and acquired the same structures 

 as modern glassy lavas, but that the lapse of time 

 has devitrified the matrix, or induced the crypto- 

 crystalline structure and altered the general appearance 

 of the rocks. Figs. 82, 83, 84, 85, and 86 are sections- 

 of tertiary pitchstones from Meissen and Kremnitz, 

 and of Pebidian felsites, or devitrified pitchstones 

 from Wrockwardine, to show how nearly the por- 

 phyritic, microlithic, perlitic, and spherulitic struc- 

 tures correspond in rocks of such widely different 

 ages. 



The age of these Wrekin rocks is well shown, first, 

 by their unconformable junction with the Hollybush 

 sandstone ; and second, by the occurrence of pebbles 



N W. 



Fig. 8?. — Section across the Wrekin Chain (Callaway). 1, Bedded Pre-Cambrian tuff, dip N. 2, Quartzite. 

 3, Hollybush Sandstone. 4, Sheveton Shales (Tremadoc). 



Fault. 



Fig. 81. — Section along Wrekin Chain (Callaway). 



Fault, 



and indicate that they are caused by the strain of the 

 rock in contracting during solidification. The ra- 

 dial structure of the spherulites is well seen. Now 

 the streams of microliths are seen to avoid the 

 porphyritic crystals and flow round them (fig. 85), 

 but to run in a straight course through the sphe- 

 rulites (figs. 85 and 86) and through the perlites, 

 indicating clearly enough the order of events during 

 the consolidation of the rock ; viz. that the porphy- 

 ritic crystals must have been formed first, then that 

 the microlithic fluxion structure was caused by the 

 rock flowing in a viscous state, and lastly the rock 

 during contraction and cooling formed perlites and 

 spherulites. These felsites in general appearance 

 are not at all like any modern volcanic rocks, but 

 these microscopic structures are found exactly re- 

 produced in tertiary volcanic rocks, such as pitchstones 

 and trachytes, and only in them ; while in every case 

 which has been examined the order of events during 



of the Wrekin felstone in the Cambrian conglomerate 

 of Haughmond Hill. 



The intrusion of felsite of the Pebidian character 

 into the granitoidite of the Ercal Hill furnishes 

 additional proof of the relative ages of the granitoid 

 series and the volcanic series. The gneiss and- 

 granitoid series is correlated with the Dimetian or Mal- 

 vernian system, the volcanic series with the Pebidian. 



6. Charmvood Forest. — While the discovery and 

 elaboration of the pre-Cambrians of Wales was 

 rapidly progressing, Professor Bonney and Mr. Hill 

 of Cambridge were patiently and laboriously working 

 out the details of the anomalous rocks of Charmvood. 

 They have at last made out the succession in this 

 complicated district, and by the lithological and 

 microscopical characters of the rocks they have 

 correlated them with the Pebidian of other areas. 

 The principal rocks are slaty and gritty beds, thick 

 masses of agglomerate of a rhyolitic (felsitic) type, 



