446 



MR. A. HAEKEE ON THE EEUPTIVE E0CE:S LN" THE 



which he speaks of as the " Rhos Hirwain * sj^enite," is of Pre- 

 Cambrian age. Mr. Tawney apparently regarded this theory -with 

 considerable doubt, but Professor Blake f, if a passing allusion of 

 his is correctly understood, endorses it. 



An examination of the whole district leads to a conclusion in 

 favour of the earlier view, that the granite is intruded through the 

 shales. A good section is offered by the most southerly of the 

 quarries near Cefn-amwlch. This was figured by Mr. Tawney, who 

 was driven to some very unnatural suppositions in order to reconcile 

 it with the supposed Archaean age of the granite. I give, in fig. 2, 



Pig. 2. — Section in tlie most southerly Quarry at Cefn-amivlch. 

 (Length about 30 yards ; vertical scale slightly exaggerated.) 



N".2f.W. 



S.S.E. 



S. Shah 



S'. Indurated and spotted Shale. SS. Sandstone. 



G. aranite. //. Faults. 



a different version of the section. The actual contact of the two rocks 

 is easily found, and the granite is seen to send out little tongues 

 between the laminae of the shale. Specimens of the latter rock, in- 

 durated and firmly adhering to the granite, may be obtained. At 

 the back of the quarry the shale is clearly altered, and exhibits the 

 little spots and nodules supposed to represent the incipient develop- 

 ment of chiastolite. Another quarry, well within the boundary of 

 the granite, shows entangled masses of baked shales. This is partly 

 due to faulting ; but here, too, good junction-specimens of the two 

 rocks were procured. The granite in these quarries is quite typical 

 of that which forms Mynydd Cefn-amwlch, and extends towards 

 Sarn, Bryn-croes, and Llangwnadl. 



At Meillionydd the junction is not exposed ; but the quarry must 

 be near the boundary, and the presence of fragments of indurated 

 shale included in the granite is suf&cient proof of the intrusive cha- 

 racter of the latter rock. 



The granite is therefore of more recent age than the Upper Arenig 

 strata. To assign an upper limit to the date of its intrusion is a 



^ Ehos Hirwain is the low-lj-ing tract to the west, and the conjunction of 

 this name with the word Syenite on the Survey map is apparently uninten- 

 tional. 



t Eeport Brit. Assoc. (Birmingham meeting) 1885, p. 669. 



