524 MISS CKOSFIELD AND MISS SKEAT ON THE [Aug. 1896, 



The literature relating to this district is as follows : — 



Davidson, T. 'British Brachiopoda,' vol. iii. pp. 232, 233, pi. xxxiii. figs. 17, 21, 

 Memoies op the Geological Survey, vol. i. (1846) p. 25 & vol. ii. (1848) pt. i. 



pp. 262 et seqq. 

 Mtjkchison, R. I. ' The Silurian System ' (1839), pt. ii. pp. 355, 358, 361, 664 & 

 • 720, pi. xxv. figs. 3 a, 3 b & pi. xxxiv. fig. 9. 



. ' Siluria,' 5th ed. (1872) pp. 55-58. 



Salter, J. W. 'A Monograph of the British Trilobites' (1864), p. 173, pi. xviii. 



fig. 11 & pi. xxv. fig. 4. 

 Stmonds, W. S. ' Records of the Rocks ' (1872), pp. 99, 100, 125. 



The district was geologically surveyed by Sir H. De la Beche 

 and Mr. W. E. Logan in 1845, with additional Silurian lines by 

 Mr. W. T. Aveline in 1855-56, and is shown on Sheet 41. 



II. Physical Structure. 



The physical structure of the country is sufficiently marked to 

 demand a slight description. The most striking features are the 

 isolated hills with their steep sides, abrupt squarish outlines, and 

 flat tops sloping westward, as, for example, Llangunnor and* 

 Trevaughan Hills, and Bryn Merthyn. These hills are part of the 

 system of roughly parallel ridges which cross the country from 

 north-east to south-west. The practically uniform level of the hill- 

 tops, which range from 400 to 500 feet in height, and the presence 

 of the common westerly slope suggest that we have here an ancient 

 plain of marine denudation. The axes of the hills are formed of 

 bedded grits, sandstones, or slates in a vertical position, while the 

 valleys consist of the softer shales. This structure is primarily due 

 to the earth-movements to which this district, like the rest of 

 South Wales, has been subjected, but the processes of denudation^ 

 have emphasized it and laid it bare. The streams flowing east and 

 west have worn away the softer shales, forming valleys with gently 

 sloping sides; the dip-valleys, on the other hand, are strikingly 

 deep and narrow, and the streams have cut out steep and rocky 

 gorges. The river Towy enters the district from the east, but 

 makes a sharp southward bend near the town, where it is joined by 

 its tributary the Gwili ; the united streams then take a remarkably 

 sinuous course, and have covered the broad level valley with a thick 

 deposit of alluvium. It seems probable that the river-bed marks a 

 line of considerable geological disturbance. The district is bounded 

 on the south by the long low range of Old Red Sandstone hills, and 

 the undulating slopes of these present a marked contrast to the 

 often abrupt outlines of the hills formed of the older rocks. The? 

 earth-movements which have affected this district are so compli- 

 cated that it would be useless to attempt to give any exact idea of 

 their effects within the limits of this paper. They have resulted in 

 the formation of innumerable folds and faults, the latter of which 

 are most noticeable in the north just outside our district. The 

 distribution of the folds makes it possible roughly to distinguish two. 

 great periods of earth-movement. The first period gave rise to an 

 immense number of small anticlines and synclines, the axes of which, 

 run north and south. In consequence of this folding the beds, 



