246 



EEV. J. F. BLAKE ON" THE KOCKS MAPPED AS 



junction becomes for some distance a natural one ; but on descending 

 into the valley of Llanberis we find the immense masses of grit 

 which overlook Marchlyn Bach are suddenly reduced to a breadth 

 of outcrop of less than 150 feet, while on the opposite side of the 

 valley at least 600 feet are seen in continuous sequence on the 

 precipitous face of Clogwyn Mawr (=Pen Careg-y-Pran). It is true 

 that Sir A. Eamsay says " it requires much dissection of the strata 

 to make out that the crags of Pen-Careg-y-Eran . . . are all repre- 

 sented by the narrow strip of grit that crosses east of the slate 

 quarries,"^ but no amount of dissection can make a continuous 



of 600 feet reduce to an outcrop of 150 feet in a quarter of 

 a mile. A great fault down the Lake of Llanberis now throws the 

 junction 1^ mile to the west. Erom Llanberis town in a south-east 

 direction the evidence for a fault is more theoretical, but the line of 

 junction is a very straight one, and as it rises nearly 1000 feet in 

 less than a mile, the plane of separation must be nearly vertical ; 

 yet the dip of such grits as are seen is only 20° to the E. The beds 

 on the east of the line, by Bettws Garmon, have quite a different 

 aspect from the BronUwyd Grit, and brown slates are close at hand. 

 All along the east side of MoelTryfaen the smooth and marshy ground 

 seems to show the absence of any grit, and on descending the valley 

 leading to NantUe one sees that the rocks nearest the slate quarries 

 are black Silurian shales. South of NantUe Lake the line of junction, 

 which continues remarkably straight — though passing over hill and 

 dale, while the Silurian beds have no higher dip than 60° — intersects 

 two quarries. In the first (fig. 2) massive grits are in contact with 



Eig. 2. —Fault in Quarry, S. of Nantlle (looking S.S.E.). 



a. Purple slates. 



grits. 



the Purple Slates ; in the adjoining quarry dark slates, whiJe in both 

 the fault is seen, for the beds are truncated at the surface of junction. 

 The massive grit which crops out on the hillside soon disappears, 

 having been brought in by a synclinal ; and dark shales continue on 

 the Silurian side. Einally, continuing for little more than half a 

 mile the line which now runs nearly west, we reach the great 



Mem. Geol. Surv. toI. iii. 2nd ed. (1881) p. 174. 



