ORIGIN OF THE ROCKS AND SCENERY OF NORTH WALES. 215 



Previous to the year 1831, the rocks of North Wales were grouped 

 together under the very general and somewhat meaningless term of 

 greywacke, and nothing like a regular order of succession was known to 

 exist. In that and the following year, Sedgwick, assisted by Charles 

 Darwin, examined the rocks which occur along a line passing S.E. from the 

 Menai Straits, and discovered that the principles of stratification, which 

 had been elaborated by William Smith and other pioneers of geology, 

 with special reference to the secondary formations, were equally 

 applicable to the primary formations ; although, owing to their greater 

 age, they had been far more affected by the agents which contort, fracture, 

 and alter the rocks of which the earth's crust is composed. 



The order of succession established by Sedgwick during the two 

 years previously mentioned, has remained undisturbed in all its essential 

 characters to the present day, and although the name of Cambrian, which 

 he subsequently applied to these formations, is not the one usually 

 adopted, yet to him belongs the sole credit of determining their order 

 of succession in the first instance. 



My object this evening is not, however, to present you with a history 

 of the great Cambro-Silurian controversy, but to give some account of 

 the actual structure of the district, and of the changes in physical 

 geography, which an examination of this structure reveals to the 

 geologist. 



And in the first place I propose to study the rocks apart from their 

 order of succession. A traveller in North Wales who attentively 

 examines the rocks over which he passes will soon find that, in spite of 

 innumerable minor differences, there are certain well marked types. Let 

 us select a few of these and consider them in some detail. To give names 

 to them in the present state of our science is somewhat difficult, but for 

 the purposes of description it is absolutely necessary. I will, therefore, 

 choose those names which seem to me least objectionable, and at the 

 same time ask you to remember that geologists are not agreed as to the 

 precise signification which should be attached to them. I will 

 select for the most part rocks of which I have specimens, and 

 in what I say about their origin will limit myself to these specimens and 

 to others undoubtedly similar, without pledging myself in any way as to 

 the origin of other rocks to which geologists may give the same name. 



The first rock I propose to consider is one to which the term quartz- 

 felsite is now very generally applied. On the north and south shores 

 of Llyn Padarn, the lower of the two Llanberis Lakes, as well as in other 

 regions.there occurs a peculiar rock to which the geological surveyors applied 

 the term quartz porphyry. I have two specimens, one from the north shore 

 of Llyn Padarn, the other from a quarry not far from Cwm-y-glo Railway 

 Station. On the map this rock is represented by a deep red colour, and 

 the mass to which it belongs trends N.E. 'and S.W., and is evidently 

 continued beneath the lake. It is described in a paper by Professor 

 Bonney, published in the Q.J.G.S., Vol. XXXV., page 312, but I will give 

 the description direct from my own specimens. 



