C A. Matley — The Arenig Uocks near Aberdaron. 119 



attempt would meet with some measure of success. Fossils, though 

 far from common, are not so rare, even in the Arenig beds, as has 

 been generally supposed, and sufficient have now been found in 

 the small area at present touched upon to show that it contains 

 beds of Lower, Middle, and Upper Arenig age. The writer hopes 

 to return to the district and continue the examination of the area, 

 but meanwhile the following preliminary account summarizing 

 the results so far attained may be of interest. 



Area examined. 



The Ordovician ground examined is a tract about four miles wide 

 around the shores of Aberdaron and Llanfaelrhys Bays. A projecting 

 outcrop of pre-Cambrian rock divides this area into two parts, 

 a western portion abutting on Aberdaron Bay and an eastern portion 

 adjoining Llanfaelrhys Bay. 



Western or Aberdaron Tract. 



The Ordovician beds here, except where bounded by the sea, are 

 limited in all directions by faults which throw them against the 

 pre-Cambrian rocks of the district. The general structure appears 

 to be a syncline. Much of the ground is covered by thick masses 

 of glacial drift. 



Grit and 



breccia 



W Picrite 



Mynydd Parwyd 



Bychestyn 



Pen y Cil 



Section near Pen y Cil. 



4. Drift. 



3. Dolerite Sill. 



2. Arenig rocks. 

 1. Pre-Cambrian. 



/. Pault. 



W 



2 Grit 3 



and breccia 

 Fig. 2. — Section at Porth Meudwy. 



On the western side of Aberdaron Bay we have the following 

 sequence in descending order, as shown in Figs. 1 and 2 : — 



(3) Dark shales surmounted by a thick sill of dolerite (Pen y Cil dolerite). 



(2) Grit and breccia, forming a zone about 60 or 70 feet thick. The breccia 



contains abundant fragments derived from the pre-Cambrian region to 



the west. 

 (1) Dark slaty shales, usually hard and sandy, with numerous courses of fine 



sandstone. 



