J 



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11 



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i 





i 





i ml 



i be: 

 mill 



flipr 



.... 



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AND SHROPSHIRE. 



177 



part of which an outlier of felspathic porphyry rests to the 



[ant-y-Mor. This outlier belongs to the felspathic 



of 



w hie h are 



of 



of 



later 





Mowddwy. 



The Snowdon trap lies on highly fossilliferous sandstones, 

 and the solid mass that forms the base of Snowdon splits 

 into several large divisions as it passes northward to Y 

 Glyder-fawr (section i). The Snowdon mass is, therefore, 

 the result of several eruptions. On the top of the moun- 

 tain are ashes, sometimes very solid and felspathic, at other 

 places calcareous and fossilliferous. 



As a whole, the North Wales rocks show two principal 

 epochs of eruption, the first indicated by the rocks of Aran 

 Mowddwy {section 7), Cader Idris (section s), Arenig (sec 

 tion 9), and Moelwyn ; the second, by Snowdon (section 4) 

 and Y Glyderfaicr (section 1). In No. 4 they occur in the 

 same section, showing a clear order of superposition. It is 

 worthy of remark that, the interbedded felspathic igneous 

 rocks of Moelwyn, dipping under the rocks of Cynicht and 

 Snowdon on the louth, do not rise on the north side of the 

 Snowdon synclical axis (or basin) between the mountain 

 and the Cambrian rocks near Y Tryfau. They, therefore, 

 thin out between Moelwyn and Snowdon deep under the 

 surface. 



The eruptive felspathic and quartz porphyries and 

 6yenites (see p. 199) always lie amid rocks deep under the 

 contemporaneous felspathic traps and ashes, and it is pro- 

 bable that they indicate parts of the great masses of deep- 

 seated melted matter or volcanic centres, that lay much 

 deeper than the craters, but from which the melted rocks 

 and ashes proceeded that were ejected from these vents. 



Though all these rocks are strictly volcanic, it is not to 

 be supposed that any trice traces of volcanic craters now 

 remain. The whole country has been so much disturbed 

 by subsequent contortion of that part of the crust of the 

 earth, and it has been besides so long and so often subject 



Upper 

 Galleey. 



Wall-case 4. 



