THE LOWER-SILURIAN DISTRICT OF SHROPSHIRE. 459 



fragments of pitchstoiie, in some of which small crystals of orthoclase 

 and plagioclase are surrounded by streams of microliths, while others 

 have a felsitic or crypto- crystalline texture and are crowded with 

 minute crystals of felspar. A few fragments of spherulites have 

 been detected ; and there are also great numbers of broken felspar- 

 crystals scattered through the mass ; some of the thin layers are, in 

 fact, almost wholly composed of abraded felspar crystals, with small 

 fragments of microcrystalline trap disseminated among them. 



At one time some of these ashes must have been slightly vesicular ; 

 for they now contain many cavities whose walls are coated with chlo- 

 rite, and the interior filled with crystalline quartz. In a few cases cal- 

 cite and quartz are associated together. There are also some interest- 

 ing examples of microscopic cavities filled with calcite and epidote ; 

 bright yellow prisms of the latter project from the walls and cross 

 each other in various directions, the intervening spaces being filled 

 in with the calcite. Pale lemon-coloured epidote is by no means 

 uncommon in the coarser ash-beds ; it usually occurs in fan-shaped 

 groups of flat prisms, which exhibit delicate bright tints in polarized 

 light. In all the descriptions of the optical characters of this 

 mineral which have come under my notice, it is said to be strongly 

 dichroic ; this, however, is certainly not the case with any of the 

 numerous examples of pale yellow epidote which I have observed 

 in these rocks, and in the altered syenites of Leicestershire and 

 other localities. 



The masses of rock here described represent a portion only of a 

 series of similar products which have been erupted along an old line 

 of volcanic action ; porphyrinic and other varieties occur at Charlton 

 Hill and Caer Caradoc near Church Stretton : while in the hilly 

 district to the west there are still traces of old volcanic vents, accom- 

 panied by a most interesting variety of basic and acid lavas, which I 

 hope to describe on a future occasion. 



Conclusion. 



The results arrived at in the present memoir may be briefly 

 summed up as follows : — 



1. The highly characteristic internal structure of some of the rocks 

 affords the clearest proof of their original vitreous condition ; for 

 the peculiar perlitic and spherulitic formations, with their associated 

 microliths, have never been observed except in connexion with the 

 obsidian or pitchstoiie varieties of volcanic glass. 



2. It appears also that, in the older as in the younger series, there 

 is the same gradation between the vitreous and stony varieties ; and 

 as the perlitic and other glassy rocks are well known to besubaerial 

 volcanic products, the rocks here described afford strong evidence 

 that during the earlier geological periods volcanic action was of the 

 same kind and produced the same results as in more recent times. 

 I will only add, in conclusion, that probably no one who exa- 

 mines a good series of the Schemnitz rocks, or the beautiful 

 rhyolites of the Euganean Hills, will fail to recognize among 



