ANCIENT VOLCANIC ROCKS. 51 



dine, nepheline, and accessory constituents, with some 

 residual glassy base. 



As regards nepheline, this mineral, like some others, is 

 known in small perfect crystals and in larger and more 

 shapeless ones with numerous inclusions (elseolite), and 

 these two varieties are now recognised as characterising, 

 not "younger" and "older" rocks, but volcanic and plu- 

 tonic rocks, respectively. 



Again, it has been alleged that the sanidine variety of 

 orthoclase and the microtine varieties of the plagioclase 

 felspars are restricted to Tertiary and Recent volcanic rocks. 

 Sanidine is not very strictly defined, and the name is often 

 used to imply merely a fresh glassy appearance, the loss 

 of which in the felspars of the older lavas is easily explained 

 by incipient chemical decomposition. In some instances, 

 however, undoubted Palaeozoic rocks contain felspar show- 

 ing the characteristic crystal -habit, the orthopinacoidal 

 cleavage, and the glassy lustre of typical sanidine, and the 

 distinction is clearly one upon which no stress can be laid. 



It is, of course, well known that many of the Tertiary 

 lavas contain a considerable amount of isotropic glassy 

 base, and some of them (obsidians) consist almost wholly 

 of glass. The supposed absence of these glassy types 

 among the older formations was pointed out by those who 

 maintain an essential distinction between the pre-Tertiary 

 and the Tertiary lavas. The answer was given by Allport, 

 to whom in the first place belongs the honour of clearly 

 seeing and upholding the essential identity of corresponding 

 volcanic rocks of all ages. In the group of volcanic rocks 

 near Wellington in Shropshire, now known to be of pre- 

 Cambrian age, he pointed out (1877) all the characteristic 

 structures of the fresh Tertiary rhyolites of Hungary, etc., 

 still evident in rocks which have lost their glassy nature by 

 molecular changes. These ancient obsidians and rhyolites 

 show, in different examples, trichites, perlitic cracks, 

 spherulites, etc., and they are accompanied by rocks which 

 are recognised as altered volcanic ashes. Rutley and others 

 have shown that similar devitrified acid lavas have a very 

 wide distribution in the Ordovician of North Wales, 



