74 PEOF. A. H. GEEEN ON A SECTION NEAE LLAIifBEEIS. 



10. I!^0TE on a Section near Llanbeeis. 

 By Professor A. H. Geeen, M.A., E.G.S. (Eead December 3, 1884.) 



Those geologists who hold that the quartz-felsites between Llanberis 

 and Caernarvon are of Pre-Cambrian age, rest their belief mainly 

 on the fact that the conglomerate which they take to be the base of 

 the Cambrian rocks in that district contains numerous pebbles of this 

 felsite. It would be very desirable to have this inference confirmed 

 by direct evidence of stratigraphical unconformity between the two 

 rocks ; but I do not know that any one has been able to point to 

 a section where that conglomerate is seen actually resting on the fel- 

 site, and where there is an unconformity at the junction. The sec- 

 tion which I wish to describe, and which seems to have escaped the 

 notice of the geologists who have from time to time during the last 

 few years reexamined the district, does show the conglomerate rest- 

 ing with the most marked unconformity on a lower group of rocks ; 

 and it is for this reason that I call attention to it. I saw the section 

 first in the summer of 1880, and visited it again during the summer 

 of the present year. 



The section is found in one of the cuttings of the railway which 

 runs from the Dinorwig quarries along the north-eastern shore of 

 Llyn Padarn ; to show its position with respect to the adjoining 

 rocks, I have plotted, in fig. 1, the section along the railway from 

 some distance on either side of it. We start at the south-eastern 

 end in the bottom beds of the slaty series which here forms the middle 

 member of the Harlech and Llanberis * group, and which is worked 

 in the Dinorwig quarries ; beneath this come alternations of slates 

 and grits, between 800 and 900 feet thick, dipping steadily in a 

 south-easterly direction at about 60°. Beyond these, a bed of 

 massive grit comes up ; it shows no lines of bedding, but the over- 

 lying gTits are well bedded and flatten and bend over it to the 

 north-west. Then follows conglomerate and breccia, with the usual 

 character of the basement conglomerate of the Harlech and Llan- 

 beris beds. The junction of the massive grit with the conglomerate 

 is nearly vertical, and is very likely a fault. 



A little further on, the beds roll over and the conglomerate is over- 

 lain by grits dipping to the north-west. We then come to the part of 

 the section lying between the points P and Q, to which I wish to call 

 special attention (enlarged in fig. 2, p. 76). The first rock we en- 

 counter, marked A, is fissile and slaty ; it consists of a number of thin 

 layers, nearly vertical and all. lying parallel to one another, varying in 

 colour and composition, some greenish and sandy, others smoother, 

 veryflaky , and somewhat soapy. The question arises. Are these layers 

 beds or cleavage-laminse ? The marked diff'erence between them in 

 colour and composition is strongly in favour of their being beds. Next 

 comes a band marked B. The matrix of this rock is grey, or greenish 



* To avoid the ambiguity which attaches to the word " Cambrian," I xise 

 this name for the beds called Cambrian by the Geological Survey. 



