Vol. 50.] OLDER FRAG MENTAL ROCKS IN N.W. CAERNARVONSHIRE. 599 



There are gradations, and no absolute separation exists between the 

 above-named groups, but the fact that these merely minor varieties 

 recur in rocks at different horizons is a very strong argument for 

 the view that denudation was proceeding continuously. We find 

 all the six types of material present in rocks below and above the 

 supposed break. Pour of the varieties can be recognized in every 

 slide examined, and five in nearly all the sections. There are also 

 other points of likeness in the structure of the fragments, such as 

 slight varieties of fluxional, scoriaceous, and other more microlithic 

 structures. All these characters are common to the ' earlier ' and 

 the ' later ' series. 



In certain finer-grained clastic rocks, where bands of grit 

 alternate with more compact layers, the materials are evidently of 

 similar character, although the recognition of minor varieties cannot 

 be so complete as in the case of larger fragments, and fewer slices 

 have been prepared from what necessarily would be a less interesting 

 series. Fragments, however, can be recognized of the above- 

 mentioned types (a) and (b) in slices from ' Cambrian ' and from 

 ' post-Llanberis ' rocks, while some of the structures mentioned are 

 reproduced ; the finer material also is of the kind which would be 

 formed by the further wear of similar masses. Thus these banded 

 rocks can be recognized microscopically (as we have previously 

 shown that the)' can macroscopically) both above and below the 

 supposed break. 1 Also they grade into the coarser as well as into 

 the finer layers, and thus form part of a continuous series. 



The still finer gritty or banded argillites cannot, of course, afford 

 much direct evidence. Still, those assigned to the two ages have a 

 general similarity, especially noteworthy in the seams rich in 

 angular fragments of felspar and quartz, and both frequently 

 present resemblances to the groundmass of the aforenamed coarser 

 rocks. 2 



Taking, lastly, rocks formed of more acid material, the frag- 



1 G-. — East of Llyn Padarn. Slate Railway. Figs. 1 and 2, /. ' Cambrian. ' 



H.— West of Llyn Padarn. Hillside above Tan-y-pant. Fig. 7. ' Post- 

 Llanberis.' 



I. — West of Llyn Padarn. Ditto. 



- J. — West of Llyn Padarn ; near inlet of lake. Figs. 6, b ; 7, No. 11. 

 ' Cambrian.' 



K. — East of Llyn Padarn, from syncline by slate-railway. Figs. 1 and 2, d. 

 ' Cambrian.' 



L. — East of Llyn Padarn, from syncline by slate-railway. Figs. 1 and 2, c. 

 ' Cambrian.' 



M. — East of Llyn Padarn, from syncline by slate-railway. Figs. 1 and 2, 

 b-d. ' Cambrian.' 



N. — East of Llyn Padarn, from syncline by slate-railway. Figs. 1 and 2, 

 b-d. ' Cambrian.' 



O.— East of Llyn Padarn, near the top of YBigl. Fig. 2. ' Post-Llanberis.' 



P. — East of Llyn Padarn, near top of Y Bigl. Fig. 2. ' Post-Llanberis.' 



Q.— East of Llyn Padarn, about S. W. of the top of Y Bigl. Fig. 2. ' Post- 

 Llanberis.' 



E.— East of Llyn Padarn, about S.E. of the top of Y Bigl. Fig. 2. ' Post- 

 Llanberis.' 



