Yol. 62.] TARAHNOK SEEIES OF TABA.NNON. 661 



This group of beds may be termed the Gelli Shales, from the 

 farms of Gelli-dywyll and Gelli along the western flanks of the 

 tableland, near which the shales are well-developed. 



Unlike the groups which we have described down to this point, 

 wherein the descending sequence is practically unbroken by faults 

 and folds, the rocks of this Gelli Group have been much disturbed. 

 They have been subjected to earth-crust movement from the north- 

 north- west,?resulting in the frequent repetition of many of the beds 

 by means of sharp folds. These folds are probably of small ampli- 

 tude, but their crests, which are shown at intervals in the hio-h 

 banks of the river, are very sharp, and are sometimes accompanied 

 by faults, so that the steady north-westerly dip of the beds is only 

 interrupted occasionally and that for a distance of a few yards. 



These folds in their course to the north-east cut across the con- 

 tinuation of the Talerddig Beds, and are well shown on the 1-inch 

 Geological-Survey map by the zigzag shape of the outcrops on the 

 hill above and to the east of the river. This folding prevents us 

 from making more than a rough estimate of the thickness of the 

 Gelli Shales ; but it cannot be less than 800 to 1000 feet, and may 

 be more. 



Coming now to the details of the section, so far as it is exposed 

 in the Tarannon Eiver, we find that underneath the lowest grit-band 

 of the Talerddig Group there is a thickness of about 150 feet of 

 grey shales, alternating every few inches with numerous thin fla°-s. 

 Fossils are rare in these beds, but specimens of Monograptus Marri 

 and M. nudus were obtained. Below these shales some blue-black 

 flaggy beds, exposed along their strike where the river curves 

 abruptly to the north-east, yield examples of M. Marri in great 

 abundance, together with M. nudus and M. subconicus (?). At this 

 point the left bank of the river is very lofty, and presents a 

 magnificent section of Gelli strata. Here and there an occasional 

 softer and darker band of shale, often only a fraction of an inch in 

 thickness, and generally weathering to a brilliant orange colour 

 yields graptolites ; and from one of these seams, at a point about 

 300 feet below the base of the Talerddig Group, there were 

 obtained : — 



Monograptus cris2M-s. Lapw. (f. C). 

 Monograptus exiguus, Nick, (f. C). 

 Monograptus Marri (f. C). 

 Monograptus discus (f*. C). 

 Monograptus nudus (f. C). 



Monograptus subconicus (f. C). 

 Petalograptus palmeus, var. tenuis 



Barr. (O). 

 Monograptus priodon. 



Immediately beyond this seam the beds become vertical, and are 

 bent over into a sharp fold, the southern limb of which is partly 

 replaced by a fault. The beds right themselves, however, within a 

 few yards, and return to their normal dip of 45° north-westward. 



Continuing the section, we find that the beds vary but little in 

 lithological character, although the flaggy seams decrease in number 

 and thickness. They dip generally north-westward, but every now 

 and again are crumpled and folded to a greater or less extent. 



