248 The Rev. A. Sedgwick's Outline of 



the road from Carnarvon through Llanllyfni to Tremadoc. In the 

 south-western country, the coast, from Porthdinlleyn to the end 

 of the great promontory, and to Bardsea Island, is composed of chlo- 

 rite and mica slate. It forms a band on the average not two miles 

 in breadth, but it is evidently the prolongation of a formation which 

 is widely expanded in the Isle of Anglesea. At its north end it is 

 associated with a mass of brecciated serpentine (like that subordi- 

 nate to the same rocks in Anglesea), and it is here and there pene- 

 trated by veins of calcareous spar, sometimes so abundant as to re- 

 place the ordinary rock, which in such cases passes into great irre- 

 gular masses of white crystalline limestone. Near its north end it 

 is cut through by five or six nearly transverse vertical dykes of au- 

 gitic trap of a later formation. 



The other parts of the promontory are composed of greywacke, 

 (sometimes passing into a coarse arenaceous rock), and greywacke 

 slate, often of a dark colour and rather earthy structure ; and these 

 rocks are pierced and broken through by many great bosses of sye- 

 nite (represented on a geological map), which rise into hills of re- 

 markable and irregular outline. That the syenites are posterior to 

 the slates the author shows by the evidence of sections : but the 

 slates are little altered, except near the places of contact. Their 

 prevailing strike is nearly parallel to the mean direction of the pro- 

 montory (about north-east). Near the bosses of syenite these beds 

 are sometimes almost vertical, and they contain one or two bands of 

 organic remains, the most remarkable of which are found in a ver- 

 tical ridge of coarse greywacke near Bodean*. 



In the promontory near St. Tudwal's Island the rocks are thrown 

 into low undulations, are traversed by mineral veins, and intersected 

 by one or two trap dykes. 



The author here notices five or six dykes of augitic trap which 

 cross to the west side of the Menai near Bangor. They appear like 

 great ribs locking together the mineral systems of Anglesea and 

 Carnarvonshire. Similar dykes appear in three or four places in the 

 higher parts of the Carnarvon chain ; and all dykes of augitic trap 

 above noticed are considered of nearly the same epoch, and of a later 

 date than the mountain limestone and new red sandstone of the 

 Menai Straits. 



Near the line of road above mentioned the country is at a low 

 level, and much concealed by drifted matter ; it is however marked 

 here and there by erupted masses of felstone porphyry, which in one 

 or two places are finely columnarf. Near this line are some enor- 

 mous faults which have thrown the south end of the Carnarvon chain 

 about 2000 feet above the level of the road. Near Tremadoc the 

 continuation of the same lines of fault have torn mountain masses 



* Among these fossils are, — 1. many encrinital stems; 2. Entomostraciics 

 punctatus; 3. Lcptcena sericea ; 4. Or this Flabellulum, O.pecten, 0. canalis, 

 O. testudinaria, &c. 



•f* By the word felstone the author defines many rocks commonly called 

 compact felspar ; an incongruous name he wishes to replace by a name 

 (Feldstein) occasionally used in Germany. 



