174 



IGNEOUS ROCKS OF WALES 



Upper 

 Gallery. 



Wall-case 4. 



well crystallized, as in 118, 119, 182, &c. These, and others 

 of similar kinds, occur sometimes in great masses, as in the 



), Corndon Hill (section 5), and 



Craig-das-Eithen 



(section 7). In general, however, 



long 



lines, mam/ of 



Wales and Shropshire, they run in 



which have been intruded more or less between the beds of 

 the stratified rocks.-— (See Moel Siabod, section 1 ; section 2 ; 



Cader Idris, section 



Moelwyn 



8 ). In some instances they run for miles directly in the 

 strike of the strata, and then break slightly, or in other 

 cases quite suddenly, across it. If ever, it is only in very 

 rare cases, that they are truly contemporaneous, one proof 

 of which is, that the slates or other beds which they pierce 

 are alike altered at their points of contact both with the 



rfaceof 



Had the green- 



stones been poured out on ordinary sediments they might 

 have altered the sedimentary surface over which they flowed 



the muddy sediment which fell on the 



but being cooled 



upper surface of the lava remained unaltered. This 

 the case with all the truly bedded felspathic traps, afterwards 

 to be mentioned. Another proof of the intrusive character 

 of the greenstones is that they frequently branch. They 

 are also often columnar, the columns lying at right angles to 

 the dip of the rock. In the district under review they 

 never penetrate the Upper Silurian strata, and as they gene- 

 rally partake of the curves or contortions that affect all 

 the rocks of Wales, it is inferred that they were injected 

 amid the beds before the disturbances took place that 

 produced the sweeping undulations of the strata. 



The contemporaneous igneous rocks of the district are 

 also of two kinds. 



1st. Felspathic porphyries or lava beds. 

 2nd. Felspathic and calcareous volcanic ashes. 

 The first are coloured light vermilion. The specimens 

 from Nos. 94 to 110 are characteristic of this class of rocks, 



especially those from 97 to 101. 



In 97 the porphyritic 



character is well exhibited, small crystals of yellow felspar 

 being set in a dark blue base. 98 shews the scoriaceous 



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