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



melted rock in the dykes was insufficient to produce these effects 

 of metamorphism, and that in the other instances a great body of 

 igneous rock was close at hand, affording a more intense heat ca- 

 pable of producing the modifications observed. 



3. On the Geology of the neighbourhood o/ Trem adoc, Caernarvon^ 

 shire. By J. E. Davis, Esq., F.G.S. 



The little town of Tremadoc (in the northern part of Cardigan Bay) 

 is beautifully situated on the north side of a valley formerly covered 

 by the sea. Mr. Maddock having in the year 1813 made an em- 

 bankment across the wider valley of Traeth Mawr, of which, on the 

 western side, the Tremadoc valley was an inlet, the sea was barred 

 out, the valley brought under cultivation, and the town erected. 

 On approaching this district of Caernarvonshire from the east, it 

 does not require the eye of a geologist to be assured that the sea 

 once, and at a comparatively recent period, extended many miles 

 inland. The succession of cliffs with steep escarpments facing the 

 present line of coast, and extending inland up the eastern side of 

 Traeth Mawr, one behind the other, for several miles, are striking 

 features in the scenery of this romantic neighbourhood. 



The stratified rocks of the district have a general strike from 

 north-west to south-east, with a north-east dip. They consist of 

 slates, shales, flags and sandstones, but, from the almost total abs- 

 ence of organic remains, the determination of the age of the beds 

 must be a work of considerable difficulty and time, and is one in 

 which great caution is requisite, for all these rocks have apparently- 

 undergone great change of structure in many places, and that which 

 is the same formation or bed, presented under a new aspect, may be 

 easily mistaken for a totally distinct deposit. 



Although some of the rocks are described as slates, the slaty 

 cleavage is too imperfect to admit of the rock being quarried for 

 economic purposes, and true slates are not obtained nearer than Fes- 

 tiniog, several miles east of Tremadoc, a district to which these ob- 

 servations do not extend. The strata however, whether flagstone, 

 sandstone or shale, possess in many places a distinct structure, con- 

 sisting of an irregular prismatic cleavage observable in many places, 

 the rock shivering into splinters of a foot or more in length, and from 

 one to three inches in width. These splinters conform to the direction 

 of the dip, and their points, as far as they have been observed, dip 

 at the same angle. A good example of this structure is exposed by 

 a cutting on the left side of the road leading from Portmadoc to 

 Tremadoc, and within a few yards of the last house in the former 

 place. 



The only fossils I met with were a Lingula and traces of Fucoids, 

 occurring in great abundance in flagstones near Penmorfa church, 

 about two miles west from Tremadoc. and also on the south side of 

 Moel-y-Gest. The Fucoids resemble the remains found in the 

 Upper Ludlow rock of the Silurian system, but similar species have 



