386 MISS G. L. ELLES ON THE WENLOCK [May 1900, 



northern part of the district been examined. As it is, the succession 

 is confirmed, and there can be no doubt, I think, that the Wenlock 

 Shales of the Builth district are capable of being divided into the 

 following six distinct zones (in descending order) : — 



Feet. 



(6) Zone of Cyrtograptus Lundgreni, Tullb about 440 



(5) Zone of Cyrtograptus rigidus, Tullb about 400 



(4) Z me of Cyrtograptus Linnarssoni, Lapw 1 50 



(3) Zone of Cyrtograptus symmetricus, sp. nov 150 



(2) Zone of Monograptus riccartonensis, Lapw 300 



(1) Zone of Cyrtograptus Murckisoni, Oarr. * 200 



General Sequence of Events in the Builth District. 



The general sequence of events in the Builth district is now seen, 

 therefore, to have been as follows : — 



(1) Subsidence of the old Llandeilo ridge, inducing deposition on 

 a sinking shore-line, and consequently overlap of higher beds 

 on to lower. 



(2) Production of a series of folds whose axes ran north-east and 



south-west. This set of movements was responsible for the 

 synclinal fold which occupies the northern part of the area. 

 These movements were probably of post-Silurian age, since 

 they affected the rocks up to the Ludlows, but not apparently 

 the Old Red Sandstone. 



(3) Production of a second set of earth-movements resulting in 



a series of folds whose axes ran in a general east-and-west 

 direction. These movements were probably post-Carboni- 

 ferous, though the evidence of age is not seen in this district. 



(4) Denudation of the rocks thus folded. 



IV. The Long Mountain Djsteict. 



The general structure of the Long Mountain is very simple. The 

 beds are bent into a syncline whose axis runs north-east and south- 

 west, and the rock-succession presents many features in common 

 with the Builth district just described. Here again there must have 

 been deposition in a sinking area of older rocks, resulting as before 

 in overlap ; here also, as in the Builth area, the older rocks are ex- 

 posed on the outer edges of the trough. Denudation has, however, 

 laid bare lower beds of the Wenlock Shales south of the Long 

 Mountain than any that are exposed on the north side of the trough : 

 these are to be seen near Chirbury. 



The outcrop of Ludlow rocks as indicated upon the Geological 

 Survey map is too limited, and seems to include only the Upper 

 Ludlow. Beds of Lower Ludlow age extend down the mountain- 

 slopes on all sides, while the true Wenlock Shales are in reality 

 restricted to a mere fringe, occupying the lowest ground of the area 

 (see Map, fig. 9, p. 395). 



The succession is very similar to that at Builth. The oldest 

 rocks seen on the north side of the district are a series of shales 



