376 



ME. J. V. ELSDEN OX THE AGE OF 



[Aug. 1904, 



The contacts with the felsite on each side look fairly clean and 

 regular ; but, near the centre of the exposure, which is about 10 feet 

 wide, the 'greenstone' is penetrated by a tongue of felsite, about 

 2 inches broad near the upper exposed part, and tapering to a 

 point at the lower extremity 

 2 feet. 



Che length of this tongue is about 





^ 



1 



^ 

 ^ 





-to 



A thin slice of this rock, if examined beneath the microscope, 

 presents the appearance of a quartz-felsite, and resembles in all 

 essential features the descriptions given by Prof. Bonney of the 

 felsites of this area. 1 The section shows irregular subangular 

 blebs of quartz, giving somewhat undulose extinction, and 



1 T. G-. Bonney, ' On the Quartz-Felsite & Associated Rocks at the Base of 

 the Cambrian Series in North-Western Caernarvonshire ' Quart. Journ. Geol. 

 Soc. vol. xxxv (1879; pp. 311 ct seqq. 



