102 ME. Tf. LAPWORTH ON THE [Feb. 1 9 00 ? 



(3) Exposures in Caban Coch Gorge. 



As we leave the summit, and commence to descend into the Caban, 

 we pass over the succession in descending order, repeated on the 

 north side of the syncline. The individual beds in the gorge may be 

 followed by the eye for some distance on both sides of the Elan as they 

 sweep down towards the river itself; the Lower Conglomerate 

 (Ba^ can be traced eastward to the bed of the stream; but the 

 upper divisions are truncated by a transverse fault passing through 

 the west end of Cnwch Wood. The Intermediate Shales (Ba 2 ) 

 appear to have a thickness of about 300 feet on the south side of 

 the river, and the Upper Conglomerate (Ba 3 ) somewhere 

 between 80 and 100 feet. Two quarries in the Lower Conglo- 

 merate (BaJ have been opened out/one on each side of the river. 

 These are used for supplying building-material for the masonry- 

 dams on the Elan works. The lowermost of these dams — the Caban 

 Dam — is now being built, its foundations resting on the Lower 

 Conglomerate. The excavations for the trench revealed a fine 

 section in these beds, from which, with the addition of the two 

 quarries, I have been enabled to form a very fair conception of the 

 general characters of the greater part of this lowermost division. 

 At the present time the faces of both quarries have been carried 

 well back into the sides of the hills, and two exposures of clean 

 rock are now to be seen. Prom an examination of these it may be 

 concluded that the unweathered material is extremely tough, both 

 in the grit and in the conglomerate, for the lines of fracture due to 

 blasting pass through pebble and matrix alike, and it is practically 

 impossible to remove a pebble from the matrix of the rock. A 

 blow from a hammer produces little effect, and the forcible expres- 

 sions employed by the masons while dressing the stone testify to its 

 extreme toughness and resistance under the chisel. The individual 

 beds reach 20 feet in thickness, and huge lenticular masses of 

 pebbles occur throughout. Innumerable black clay-galls and patches 

 of dark blue shale blotch the faces of the conglomeratic beds, many 

 of them reaching a length of 12 or 18 inches. At about 50 feet 

 below the top of the group is a thick zone of intermingled grits 

 and thin slate-beds. Graptolites occur in the argillaceous bands, 

 but they are few and far between, only Monocjra'ptus crenularis and 

 CUmacograptus normalis having been detected. 



Many of the slates rapidly thin out and disappear, much in the 

 same way as the pebble-beds; in fact, one gathers a general im- 

 pression that the whole group is a collection of the material of 

 successive shingle-banks which were deposited either close to the 

 shore or in some river-estuary. The beds are much folded. In 

 the dam-excavations they dip southward on the north side of the 

 Elan ; but in the quarry above they are seen to incline to the north. 

 The total thickness of the Lower Conglomerate is estimated at 

 over 200 feet in the Craig Cnwch crags to the westward. 



The Intermediate Shales have a thickness of only about 250 

 feet above the northern quarry, or 50 feet less than on the 

 opposite side of the river. It is therefore not improbable that a 

 strike-fault passes along these beds, but no sign of it is apparent 



