Yol. SS."] THE GEOLOGY OF MTNTDD-Y-GADER. 357 



{b) The Contemporaneous Rocks. 



(i) The Rhyolitic and Ashy Series. — To the south-east of 

 the Mynydd-y-Gader dolerite occurs perhaps the most varied and 

 interesting series of exposures in the district : — dolerites, nodular, 

 banded and compact rhyolites, tuffs coarse and fine, and slaty beds 

 rapidly succeeding one another. Most of these bands are, however, 

 lenticular ; and, when followed along the strike to the north-east 

 and south-west, they die out. 



In a traverse followiug approximately the line CD shown in the 

 south-eastern portion of our map (Pi. XXXV), the rocks passed 

 over are as follows : — 



(1) Andesite, a broad and prominent band ; the felspars tend to weather 



out, giving the rocks a pseudo-vesicular appearance. 



(2) Narrow band of tuif dipping at one point south 10° west at 42°. 



(3) Dolerite enclosing patches of tuff. 



(4) Black ashy shale, becoming more distinctly ashy up the hillside. 

 (6) Nodular rhyolite, a narrow band. 



(6) Tuff. 



(7) Main nodular rhyolite band. 



(8) Tuff. 



(9) Banded rhyolite. 



(10) Rhyolite, compact on the south, banded on the north with an intrusive 



tongue of dolerite (180). 



(11) Massive dolerite of Mynydd-y-Gader. 



When this series is followed westwards, first the nodular rhyolite 

 hand 5 dies out, so that bands 4 and 6 join. Parther on the main 

 nodular band (7) disappears, so that the ashy band (8) joins 6, and 

 the series consists of a great mass of banded and compact rhyolite 

 on the north, succeeded on the south by a great series of tuffs. 

 After the dying-out of the nodular rhyolite the ashy series, which 

 dips southwards at high angles, becomes very coarse and devoid 

 of the shaly matrix which characterizes it farther east. The 

 fragments in it are mainly rhyolite, but there are some of slate. 

 Although the banded character extends much farther westwards 

 than the nodular, it eventually dies out, and in the area to the 

 south-east of the little pond only compact rhyolite occurs. 



Towards the north-east also the nodular and banded structures 

 disappear, and the rhyolite becomes compact, retaining this character 

 as far as the Afon Aran. 



The Rhyolites. — For purposes of detailed description the 

 rhyolites may be divided into : — 



1. Rhyolites showing neither nodules nor banding. 



2. Nodular rhyolites. 



3. Banded rliyolites. 



Nodular and banded rhyolites are confined to the south-eastern 

 part of the Mynydd-y-Gader area. 



Rhyolites showing neither nodules nor banding. — 

 These rocks generally have a more or less flinty character, and a 



