74 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [NoV. 5, 



protecting power of the harder igneous rock. In the elevated 

 ridges of porphyry, the process of destruction and the consequent 

 accumulation of the talus is now occasionally prevented by masses 

 of slate in front of the cliff, the remaining evidence of their former 

 denudation. 



If it be assumed, as may fairly be done, that this talus dates its 

 origin or commencement of accumulation from the period when the 

 sea ceased to flow at its base, we are furnished not only with data 

 on which to found an approximation with regard to the time which 

 has elapsed since, but also with a proof of the gradual nature of 

 this change ; for if the sea were removed at one and the same time, 

 from the lowest as well as the highest ridges, the talus would be 

 equal, or at least in proportion to the respective heights of the cliffs ; 

 whereas the fact that the talus is greatest and has generally reached 

 its ultimate limit in the cliffs situated at a greater distance from the 

 sea, is evidence that the process of accumulation has been going on 

 for a longer period, and is consistent with the theory of a gradual 

 elevation. 



The evidence of the still more recent and continued elevation of 

 the coast is derived from the embankments which have from time to 

 time been made since the sixteenth century. These embankments, 

 commencing high up the Traeth Mawr, have been succeeded by 

 others lower down, and as the new were secured the older became 

 useless. It is evident that these embankments are not the sole or 

 principal cause of the sea no longer flowing within them, but that 

 the natural recession of the sea (or elevation of the land) induced 

 the inhabitants to anticipate, by the erection of earthen mounds, that 

 which would have been produced in a few years by other causes. 

 The sea-mark may be traced on the surface of the escarpments in 

 several of the islands in the Tremadoc valley, many feet above the 

 present level of high water. 



Tradition also lends its aid. From the rocky ground of Yns hir, 

 Madoc, one of the princes of North Wales, leaving his native coun- 

 try, sailed to unknown lands*. And to descend to more recent 

 times : I was informed that the parish register of Penmorfa contains 

 entries showing that a place in the parish called Y wern was once 

 a seaport, which immediately before the erection of the great em- 

 bankment was several feet above high water. 



The river, generally known to Welsh tourists as the Pont Aber- 

 glasllyn river, instead of taking its present direct course, flowed 

 round the Tremadoc valley, and the soil of the upper part of the 

 valley is composed of peat and decayed vegetable matter, probably 

 deposited from the fresh water, the spot being protected from any 

 strong tidal action. The river appears to have assumed its present 

 course before the erection of any embankment. 



The successive geological changes which these observations sug- 

 gest as having taken place in this district appear to be — 



First, The accumulation under water of sedimentary deposits, 

 containing a few organic remains referable to the Silurian period, 

 * It is on this tradition that Southey founded his poem of * Madoc* 



