598 PROF. T. G. BONNEY AND MISS C. A. RAISIN ON [NoV. 1 894,. 



conclusions is limited to rocks from the locality of Llyn Padarn 1 , 

 which Mr. Blake has definitely grouped with one of his two series. 2 



I. In the conglomerates or breccias of rather basic materials, 

 taking specimens from critical parts on both sides of the lake, 3 we 

 find these to consist of fragments of the following : — 



(a) Ferruginous andesitic or basaltic rocks, recalling sometimes the 

 character of masses described from the Lleyn. They include 



1. Fragments, where lath-shaped felspars, clear, but replaced 



by an aggregate of minute minerals, are embedded in a 

 continuous black opaque ground ; 



2. Fragments, where similar felspars occur in a ground speckled 



with opacite ; 



3. Fragments, with a felsitic cryptocrystalline ground, rich in 



ferrite. 



Some of these fragments may represent lava with flow- 

 structure. 

 (&) Andesitic fragments, containing viridite or chlorite within 

 crystals or in the groundmass : including 



4. Ferruginous rock with some viridite ; 



5. Rock with a matrix of viridite, perhaps originally a kind of 



tuff, 

 (c) 6. Andesite or felsite, deficient in iron oxide and viridite. 



1 Many more slices have been examined, and reference to other districts, or 

 to slices from other rocks in this district, would rather strengthen the case, but 

 it would extend the statement to too great a length. We may, however, add 

 that we have compared the slices of grits and conglomerates from Mr. Blake's 

 ' Cambrian ' and ' post-Llanberis ' beds in the neighbourhood of Llyn Padarn 

 with a number of those from the area between Brithdir and Bangor (Tair- 

 ffynon, etc.). Though varietal distinctions exist, to some of which attention has 

 been called in former papers, these beds on the whole bear a general resemblance 

 to the clastic rocks discussed in the present communication, and the lava-frag- 

 ments occurring in the one set can often be identified in the other. Hence it is 

 more probable that all belong to one and the same period than that they 

 represent two periods separated by a vast physical break. 



Mr. Blake, in an earlier paper (Quart. Journ. Geol. Soc. vol. xlviii. 1892, 

 pi. vi.), admits the conglomerate north of Llyn Padarn to be the equivalent of 

 the Bangor Conglomerate. (It is also referred to by one of us, C. A. Raisin, 

 Quart. Journ. Geol. Soc. vol. xlvii. 1891, p. 335.) To us, however, the former 

 seems very similar to the conglomerate immediately south of the main mass of 

 felsite on both sides of the lake, which he claims as ' post-Llanberis.' 



2 The specimens were not directly cut to establish this comparison, so that 

 the preponderance of any one group on one or the other side of the lake is ofteu 

 accidental. The references give the localities of the specimens as marked in 

 Mr. Blake's figures and maps as nearly as is possible, Quart. Journ. Geol. 

 Soc. vol. xlix. (1893) pp. 441 et seqq. 



3 A. — East of Llyn Padarn by slate-railway, ' rain-spot ' breccia underlying 

 supposed line of unconformity. Figs. 1 and 2, g. ' Cambrian.' 



B. — East of Llyn Padarn near road north of Fachwen. Fig. 2 about N.W. 

 of F in Fachwen. ' Cambrian.' 



C. — East of Llyn Padarn by slate-railway. Conglomerate overlying supposed 

 line of unconformity. Figs. 1 and 2, A. ' Post-Llanberis.' 



D. — East of Llyn Padarn. Ditto. ' Post-Llanberis.' 



E. — East of Llyn Padarn, to south-eastward of the top of Y Bigl. Fig. 2, 

 ? about due S. of 23. ' Post-Llanberis.' 



F. — West of Llyn Padarn, hillside above Tan-y-pant. Fig. 7, ? about due N. 

 of 16. ' Post-Llanberis.' 



