402 



♦ KNOWLEDGE ♦ 



[Nov. 14, 1884. 



RAMBLES WITH A HAMMER. 



GEOLOGY OF CEICCIETH AND PWLLHELI {continued). 



By W. Jerome Harrison, F.G.S. 

 IL 



P RE-CAMBRIAN ROCKS.— Of these rocks, of which 

 it is in some degree doubtful whether they are (a) 

 older than the Silurian strata which now surround them, 

 or (b) of later date than such beds, being, in fact, intrusive 

 in them, there are several examples near each of the towns 

 to which this article refers. The picturesque old castle at 

 Criccieth is built on a mass of columnar felspar — porphyry. 

 It is a compact pinkish rock, in which distinct felspar 

 crystals are here and there visible. About four miles west 

 of Criccieth, the headland of Pen-y-chain gives a most 

 interesting section. Approaching it by the shore from the 

 Afonwen side, we meet with (1) a compact felsitic rock 

 resembling that of Criccieth; (2) a dark-coloured rook, 

 such as the older geologists called " greenstone " ; (3) a 

 remarkably laminated bed, the laminic (about the tenth 

 of an inch in thickness) being alternately green and 

 white in colour (this may be a schistose gneissic 

 rock or a laminated volcanic ash ; it requires micro- 

 scopic examination); (4) another bed of "greenstone;" 

 (5) a beautiful breccia — either a volcanic agglo- 

 merate or a shore conglomerate little rolled, consisting of 

 masses of a pale felsitic rock like (1) embedded in a 

 dnrk matrix like (2). The included fragments increase in 

 size as we cross the strike of the bed from the size of a 

 marble until they are a foot in diameter, and the larger 

 the fragments are the less of the dark matrix is seen 

 between them, until at last they are almost in contact with 

 one another. Continuing in a westward direction we 

 cross (6) a platy felspathic rock, and then in a little cove 

 we find a very remarkable mass of (7) chalcedonic nodules, 

 the sizes varyiug from a fraction of an inch to several 

 inches in diameter. (Similar nodules occur in the little 

 promontory of Careg-y-defaid, two miles south-west of 

 Pwllheli.) Beyond this comes more of (8) the platy fels- 

 pathic rock, and finally (9) more of the compact felsite, 

 forming precipitous cliffs from which sea-fishing with the 

 rod may be carried on. 



All the nine varieties of rocks which we have described 

 are lumped together by the Geological Survey in their map 

 and memoir as " felspar porphyry " intrusive in Silurian 

 strata. My own opinion is that they are in part, at all 

 events, of immensely greater antiquity, and very probably 

 of Pre-Cambrian age. The fragments in the breccia bed 

 (5) must have been in their consolidated and hardened 

 state when they were broken up to form the layer in 

 which they now lie. This breccia bed is, therefore, of later 

 date than the felaites from which its materials were 

 derived. All these beds appear to be of volcanic origin, 

 and of contemporaneous volcanic action of this kind the 

 Bala beds (as those Silurian rocks are called which rest upon 

 the flanks of Pen-y-chain) of Lleyn offer few or no traces. 



Walking on four miles more — still westward — to Pwllheli, 

 a remarkable section is exposed in the deep cutting through 

 which the road passes to the little villagesof Llanov andDenio. 

 Here shivery slates of Bala age are broken through, and 

 cast into wonderful contortions by a felspathic rock which 

 forms a bold ridge several miles in length. Here it might 

 appear that we have clear evidence of a mass of molten 

 rock (for the felspathic rook is clearly of an igneous 

 character) having burst through the Silurian slates, and I 

 cannot say (in the absence of fuller investigation than time 

 permitted me to give) that such is certainly not the case. 

 But what puzzled me was the slight evidence of alteration 



afforded by the slaty strata. They did not appear to be 

 more indurated near the junction, and fossils could be 

 found in them quite close to the felspathic rock. It is, 

 then, possible that the disturbed junction between 

 the two rocks may be a line of fault, and that the 

 felspathic rock, already in a hardened state, was forced, 

 like a wedge, through the newer overlying slates. The 

 occurrence in this cutting of a brecciated bed, in which 

 fragments of a pale rock were embedded in an 

 ashy matrix, a bed which apparently belonged to 

 the felspathic rock, strengthened this belief. In this 

 case it is probable that all this ridge of hard rock 

 is of the Pre-Cambrian age. At the south-west end of 

 Pwllheli, near the old turnpike gate, there is an exposure 

 of a volcanic ash or breccia, which may be a continuation 

 of the breccia-bed just named. Prof. Ramsay writes that 

 it is " the only rock in Lleyn of an ashy or brecciated 

 character ; " but in view of the magnificent breccias at 

 Pen-y-chain and Porth-dinlleyn this is certainly an error. 

 The other masses of igneous rock north and west of 

 Pwllheli and Criccieth — coloured red on the Survey Maps, 

 and considered by the professional geologists who made 

 those maps as intrusive masses in the Silurian strata — all, 

 or nearly all, are claimed by Dr. Hicks as Vielonging to his 

 immensely old Pre-Cambrian rocks. But if these hills 

 — for the hard, igneous masses all stand up boldly 

 above the softer Silurian slates — are Pre-Cambrian, 

 where are the Cambrian strata, which ought to lie betvjeen 

 the Pre-Cambrian beds and the SOurian strata ? Dr. Hicks 

 accounts for the absence of Cambrian strata round Mynydd- 

 y-cennin, Yr Eifl, the Nevin, and Boduan Mountains, &c, 

 by calling in the aid of faults, by which, he says, the 

 Pre-Cambrians have been thrust upwards while the Cam- 

 brians are left below. This may be, and no doubt very 

 frequently is, the case. A further explanation may be 

 found in the overlap of the Silurian beds which has some- 

 times covered over and hidden from view the junction of 

 the Cambrian and Pre-Cambrian strata (Fig. 1). Our 

 knowledge of the Pre-Cambrian beds of Lleyn is, however, 

 as yet extremely small, and much work must be done before 

 their true nature, extent, and relations are made clear. 



Fig. 1. — Overlap of Silurian strata (3, 3) by which they abut 

 npon the Pre-Cambrian rocks (1) at x , thereby entirely hiding from 

 view the Cambrian beds (2) . 



The landlocked harbour of Pwllheli owes its existence 

 to the great boss called the "Gimlet Rock" (Welsh, 

 Carregy-rimbill), which lies at the entrance to the harbour, 

 and serves the purpose of a pier or breakwater. It is a 

 very handsome rock, a diabase, composed of the minerals 

 felspar (white) and augite (black). Beautiful crystals of 

 quartz occur in the fissures by which the rock is traversed. 

 There ore large quarries here whence excellent paving-setts 

 are obtained, and similar quarries may be seen at many 

 points round the coast, as at Llanbedrog, Nevin, and 

 Porth-y-nant. The stone is largely shipped to Liverpool, 

 but at present the demand for it is but small, and several 

 of the quarries have ceased working. 

 (To he continued.) 



