Yol. 50.] OF THE NEIGHBOURHOOD OF BUILTH. 577 



VII. Conclusions. 



There remain now to be considered the age and order of eruption 

 of these igneous rocks. As already pointed out, the fossils at present 

 found in the andesitic ash are not sufficient to fix its age, but since 

 it is overlain by Llandeilo Shales containing Ogygia buclii, etc., it 

 may be regarded as probably of Lower Llandeilo age ; so also may 

 the underlying andesites. The diabase-porphyrite and the rhyolites 

 are somewhat later than these, but at present there is no evidence 

 to show whether the rhyolites are earlier or later than the diabase- 

 porphyrite ; from theoretical considerations one would expect them 

 to be earlier. 



The diabases are of later date than the other igneous rocks, since 

 in all cases they are intruded into the Llandeilo Shales ; but nowhere 

 do they pierce the Silurian beds (Llandovery and Wenlock), showing 

 that they are of post-Llandeilo and pre- Silurian age. This is well 

 supported by the section exposed in the quarry next Pen-cerig Lake, 

 where the diabase is seen in contact with both the Llandeilo Shales 

 and the Llandovery Beds : the former are metamorphosed, the latter 

 quite unaltered. 



The order of eruption, then, in this district was probably as 

 follows : — (1) Andesites ; (2) Andesitic Ash ; (3) lihyolites ; (4) 

 Diabase-Porphyrite ; (5) Diabase. The first four are of earlier date 

 than the Upper Llandeilo Shales, the last one later. 



I must here express my thanks to my friends Mr. J. E. Purvis, 

 B.A., and Mr. H. Brownsword, B.A., for their kindness in deter- 

 mining the silica-percentages given in this paper. 



Discussion. 



Dr. Hicks said that he had examined the area referred to by the 

 Author, and he had come to the conclusion that the contemporaneous 

 volcanic rocks were quite at the base of the Llandeilo Series, and 

 mainly associated with the Llanvirn Beds — as in Pembrokeshire and 

 North "Wales. 



Mr. W. W. Watts wished to ask whether there was any strong 

 reason for placing the andesitic and rhyolitic ashes in the Llandeilo 

 Series rather than linking them with the Arenigs. He pointed out 

 that, while the general characters of the rocks agreed closely with 

 those of Shropshire, the diabases in the latter county were in- 

 dubitably of post-Silurian age. 



