ROCKS IN N.W. CyVERNARVONSHIRE. 2i3 



mate to the natural interpretation of the then known facts. New 

 tacts, more careful observations, and improved methods lead to a 

 modification of these views ; but the wholesale upsetting of their 

 conclusions by later writers seems certainly to be due to a " disre- 

 gard of the evidence by which the officers of the Survey " were led 

 to adopt them. 



Bangor and Caernarvon Area. 



It will be well to begin our examination at Caernarvon, where 

 matters are simplest, and where we may find a key to interpret the 

 rest. The " field- quarry" at Twt Hill has been the scene of much 

 controversy, and two interpretations now stand before us for choice. 

 On the one hand, Prof. Bonney declares that the conglomerate there 

 seen rests on the " granitoid rock," and cannot be younger than 



Fig. 2. — Section at Twt-Hill Field-quarry, Caernarvon. 

 S.E. N.W. 



1. Quartz-porphyry. 3. Conglomerate. 



2. Finer band of the same. 4. Grit. 



basal Cambrian ; on the other hand, the Survey make this conglo- 

 merate Ordovician, and the '' porphyry " intrusive. I have examined 

 this quarry many times, with the care that is due to its importance. 

 (The section from S.E. to N.W. is given in fig. 2.) To my eyes 

 the conglomerate does not rest on the " granitoid rock," but the 

 porphyry cuts across the edges. A knife can be inserted into the 

 plane of junction. The edge of the bed in contact (not its base) is 

 discoloured for about half an inch, suggesting the action of heat, and 

 the porphyry for the nearest foot from the junction-plane is of much 



Fig. 3. — Plan of junction of Porphyry and Conglomerate. 





1. Conglomerate. 2. Finer band of porphyry. 



finer texture, insomuch that I at first took it for a dyke. This suggests 



ore rapid cooling at the edge. Again, the surface of junction is 



not plane or undulating, but has in one place a step in it (see fi^. 3). 



