Lower Palceozoic Bocks of the Soiith-East of Ireland. 567 



deposition of the Lingula beds over the area which is now contained in the 

 counties of Carnarvon and Merioneth.* 



II. Igneous Rocks associated with the two Formations mentioned above. 



Although the Cambrian formation of our district contains large exhibitions of 

 quartz rocks, which are believed to be sandstones altered by heat (whether dry 

 or accompanied by water or steam), it is remarkable that it hardly ever has any 

 true igneous rock associated with it. 



A boss of epidotic greenstone a little south of Roundwood; a small vein of 

 highly crystalline hornblendic greenstone at Graystones, county of Wicklow; a 

 similar one at Cahore Point, county of Wexford; and a few small veins or dykes 

 of greenstone about Howth Hill, are the only igneous rocks that have been ob- 

 served in the Cambrian areas of our district. 



No locality, either, is known in our district where Cambrian rocks can be 

 seen in immediate contact with granite, nor even in very close proximity to it. 



The case is very different with regard to the Cambro-Silurian formation. 

 It has been already said that this contains a large assemblage of contempora- 

 neous traps. These are principally felstones; they ai'e known to be contempo- 

 raneous partly by their bedded character and their analogy with contempora- 

 neous traps elsewhere, partly by their being always associated with large masses 

 of " ash," which occasionally become brecciated or conglomeritic, containing 

 small and large, angular and rounded, fragments of trap and of slate, embedded 

 in a fine, sometimes pulverulent, felspathic base. 



There is, indeed, almost every gradation to be found, from a black slate into 

 a gray, from that into a pale gray or green slightly felspathic slate, with a soapy 

 feel; from that into a fissile " ash" or slate composed entirely of felspathic ma- 

 terials (with or without fragments of other rocks), and from that into a hard 

 compact felstone, with shining facets of felspar crystals; and thence into a crys- 

 talline felstone, in which crystals of both felspar and quartz make up the mass 

 of the rock. 



This latter highly crystalline felstone may often be intrusive, or partly intru- 

 sive and partly contemporaneous, and there are somewhat similar rocks which 



* See Paper in vol. Ti. of "Journal of Geological Society of Dublin," by Messrs. Jukes and 

 Wtley, " On Structure of North-Eastem Part of the County of Wioklow." 



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