1845.] DAVIS ON THE GEOLOGY OF TREMADOC. 7l 



been observed near Middleton chapel on the Comden mountain in 

 Shropshire, occurring with Asaphus Comdensis, which is believed 

 to be referable to tlie Llandeilo flags. These fossiliferous beds lie 

 below the other stratified deposits of the neighbourhood. 



The igneous rocks of this district may be divided into two classes. 

 The first, consisting of a porphyritic rock, is the most extensively 

 developed, and is intimately connected with the physical aspect of 

 the country. This porphyritic rock occurs in large masses, forming 

 elevated and parallel ridges extending from north-west to south-east. 

 The most southern of these ridges in this neighbourhood forms 

 the mountain called Moel-y-Gest, the southern boundary of the 

 Tremadoc valley, which it separates from the sea. A second more 

 prolonged ridge extends from near Penmorfa, and, forming the north 

 side of the valley and the ridge called Yr Alt Wen, terminates 

 abruptly at Tremadoc. A third ridge runs north of the last, and 

 extending still further east, terminates in the valley of Traeth Mawr. 

 Other ridges occur to the north, and their eastern extremities over- 

 hang the road leading from Tremadoc to Beddgelert. 



A columnar structure is discernible in many places, varying in 

 form and number of angles, but always on a large scale. Wedge- 

 shaped forms may be observed in the cliff" behind the inn at Tre- 

 madoc. Two of these wedges form a cube, and where a number 

 of these have given way and fallen from the face of the cliff", a 

 smooth surface is presented, having the appearance of a plane of a 

 highly inclined and nearly perpendicular stratum. Westward of 

 this spot the columnar structure is more readily traced. 



The eruption of these rocks was subsequent to the consolidation 

 of the adjacent stratified deposits. This is clearly proved by the 

 effect of their intrusion upon the adjoining strata, and by the po- 

 sition of the beds near and at the points of contact. A few yards* 

 east of the inn at Tremadoc, and some hundred feet above the road, 

 the beds of coarse slate or flagstone are nearly vertical. In Moel- 

 y-Gest beds of slate may be seen jammed in between two masses 

 of basalt, and dipping at a very great angle to the east. In the 

 ridge of Yr Alt Wen the intrusive rock appears to have been forced 

 up between beds of sandstone ; and it is singular, that while the 

 lower beds do not appear to have been materially affected, the su- 

 perincumbent mass is much altered, and assumes the splintery struc- 

 ture before noticed. In flagstones also, reposing on the north side 

 of one of these ridges, near Port Treuddyn, and adjoining the road 

 leading from Tremadoc to Beddgelert, about a mile and a half from 

 the former place, the effects of the intrusion are discernible to the 

 distance of many feet from the point of contact by discoloration and 

 partial fusion. 



It will be seen that the general strike of the stratified rocks cor- 

 responds with the line of irruption, and it seems to be a reasonable 

 inference that the latter was tlie cause of the former. At the same 

 time it is to be observed that this strike and north-east dip prevail 

 to the shore of Cardigan bay, and further south than any line of 

 igneous rock observed by me. 



