388 THE AGE OE THE LLYN-EADAKN DYKES. [Aug. I904. 



Fig. 2. Composite augite-crystal, similar to that seen in fig. 1, but showing 

 regular zones of crystalline growth. The section is parallel to the 

 orthopinacoid, and therefore extinguishes simultaneously throughout. 

 Crossed nicols. 

 o. Augite-crystal showing secondary cleavage along glide-planes. Crossed 

 nicols. 



4. Crushed diabase, showing secondary felspar, enclosing broken frag- 



ments of augite. Crossed nicols. 



5. Sheared diabase, showing abundant development of epidote and chlorite. 



Ordinary light, 

 li. The same, showing feebly double-refracting granules, presumably 

 perowskite, enclosed in chlorite. Ordinary light. 



Discussion. 



The President, while admitting that many arguments might be 

 brought forward in favour of the post-Bala age of the movements 

 referred to, also saw difficulties in this view as to their age. Among 

 these was the smallness of the unconformity between Ordovician and 

 Silurian rocks in the area to the south-east of that described by the 

 Author ; and the evidence of cleavage in the Wenlock Beds of the 

 Corwen district, comparable in many ways with that of the Cambrian 

 and Ordovician rocks of Caernarvonshire. 



Prof. Watts pointed out that the dykes described by the Author 

 resembled in many respects the sill-rocks of Shropshire and Mont- 

 gomeryshire. These rocks were probably derived from the same 

 magma as the Bala lava-flows, but they were certainly intrusive 

 into the base of the Silurian of that district as well as into the 

 Ordovician, for the basal Silurian rocks were often metamorphosed 

 at the contact. 



Mr. Fearnsides said he thought that the rocks exhibited had 

 many features in common with the basic sills which occurred 

 among the Llandeilo and Bala rocks about Tremadoc. About 

 Tremadoc many of the sills had come up along small thrust-planes, 

 and seemed to have baked rocks which, though already somewhat 

 crushed by the faulting, were still uncleaved. This being so, the 

 sills at Tremadoc must be considerably newer than the Bala Beds, 

 and should be referred to the period of Silurian and post-Silurian 

 earth-movements rather than to the pre-Silurian. 



The Author, in reply, said that, while he quite realized the 

 difficulty in assigning an exact age to the intrusions, he felt that 

 the greater the interval assumed to exist between the age of the 

 dykes and that of the sills, the more difficult became the explanation 

 of the facts adduced in the paper. The protective influence of the 

 Llyn-Padarn ridge, also, might be expected to become less marked 

 as the Cambrian sediments became more indurated, and it would 

 then prove less easy to account for the differential deformation of 

 the dykes. 



