100 ME. H. LAPWOETH Otf THE [Feb. I9OO , 



after which the group is named, its beds are exposed in mag- 

 nificent cliffs, almost vertical for some 600 or 700 feet on both 

 sides of the river ; and from the bottom of the gorge the appearance 

 of these great crags piled one above the other up to the sky-line is 

 a striking spectacle. It is, indeed, one of the finest examples of 

 rugged scenery within the district. 



(a) Typical Section at Caban Coch. 



Cnwch Hill, which lies south of the Caban-Coch gorge, is made up 

 of these Caban rocks in the form of a syncline ; taking, therefore, a 

 traverse over its heights we shall have the benefit of a repeated 

 succession. Furthermore, by extending the section across the Elan 

 to the north side of the river where the beds are again exhibited, 

 we shall cross the series no less than three times ; and in this manner 

 we ought to gain a very fair knowledge of its subdivisions. 



(1) Caban Conglomerates (Ba). 



Lower Conglomerate (##,). — Commencing our section (fig. 10, 

 p. 104) some 300 or 400 yards above Ty'n-y-graig, we find overlying 

 the Dytfiyn Flags a group of grey and bluish-grey grits and con- 

 glomerates (Ba x ). Some of the beds are very massive, averaging 

 10 to 15 feet in thickness. Freshly-fractured surfaces of the un- 

 weathered rock, and sections under the microscope, show the 

 material to be highly siliceous, with an addition of a certain amount 

 of felspathic matter. The felspar-paste decomposes generally to 

 a considerable depth, sometimes to as much as 18 inches, leaving 

 a soft, rotten, brown or ferruginous rock : from which, if it be 

 conglomeratic, the pebbles may be easily removed by the fingers. 

 The felspathic grains, in the disintegrated rock, take the form of 

 white and ochreous blehs — very prominent, as a rule, on fractured 

 surfaces — giving the rock a peculiar oolitic appearance. The grains 

 of the matrix are somewhat coarse, angular to subangular in form, 

 consisting chiefly of fragments of grits, quartz, vein-quartz, quartzite, 

 mica, slate, and felspar, with rarer pieces of acid and basic igneous 

 rocks. The pebbles are fairly well rounded ; and are, with the 

 exception of the mica, made up of the materials above mentioned. 

 The commonest pebbles are those of grit and vein-quartz. In the 

 lower beds, fragments of the former occur up to 6 inches or so in 

 diameter. The largest blocks consist mainly of slate. These are 

 found generally in irregularly-shaped masses more than a foot long, 

 but they are by no means of frequent occurrence. Next in order 

 of size and number are pebbles of white quartz, quartzite, and 

 felsite, which are rarely found to exceed 2 or 3 inches in diameter. 

 The grit-fragments closely resemble those of the Cerig Gwynion 

 Grits, but it is impossible to say definitely whether they were 

 derived from those rocks. The included slates are certainly frag- 

 ments of the underlying Dyffryn Group. They are, however, too 

 cleaved to yield graptolites. Considered as a whole, these Caban 

 Conglomerates are so distinctive in their general characters, and so 

 different from the Cerig Gwynion Grits, that the two rocks should 



