&Q 



<n 



**> r y /. 



of the igneous rock and appear to pass, 

 below it. The way in which the shales 

 curve round the northern end, and dip 

 towards the igneous mass, would suggest 

 that the porphyry is a sill-like mass 

 resting within a syncline in the Tetra- 

 graptus-shales. 



In hand-specimens, the rock is of a 

 pale-blue colour, and weathers grey and 

 white. There are many exposures of the 

 weathered rock, in the form of crags r 

 all over the hill and on the slopes ; 

 but by far the freshest rock is to be 

 seen in a small road-metal quarry im- 

 mediately north-west of Lambstone 

 Farm. 



The mass has an estimated thickness 

 of several hundred feet. From the 

 microscopic characters of this rock, the 

 absence of flow-structure, its massive 

 jointed appearance, and the seeming 

 induration of some of the surrounding 

 sediments, we conclude that it is of 

 intrusive nature. 



(4) Detailed Petrography. 



1. The Coomb Rocks. 



(a) The Lower and Upper Ande- 

 sites of the Eastern Side of 

 the Dingle. 



As there are no petrographical dif- 

 ferences to distinguish the rocks which 

 occur above and below the rhyolites on 

 the eastern side of the Coomb Dingle, 

 they may be conveniently considered 

 together. 



i) The Flows. — Specimens were 

 collected and slices cut from well-marked 

 flows— one [E 4137-38-39 1 ] 90 yards 

 south of the lower limit of the rhyolites, 

 and another [E 4130] 25 yards south 

 of the eastern tributary stream which 

 flows into the dingle at Llwyn-celyn. 

 In hand-specimens they are seen to 

 consist of coarsely-vesicular rocks of a 

 bluish-green colour, weathering brown. 



1 The numerals in square brackets through- 

 out this paper refer to the registered numbers of 

 the slides in the Geological Survey-Collection. 



