Vol. 56.] FORMATION AND ITS GRAPTOLITE-FAT7NA. 441 



with those whose sequence I worked out in the typical Ludlow 

 area, have enabled me to bring the graptolites of the two districts 

 into still greater harmony. 



Continuous sections through the Wenlock and Ludlow Beds are 

 fairly numerous in this district, especially on the northern flanks of 

 the mountain, and graptolites are abundant. It is easy, therefore, to 

 fix the position of such graptolitic zones as are recognizable, while 

 the boundary of the Wenlock and Ludlow formations may be mapped 

 with some degree of accuracy. 



The rocks are lithologically very similar to those in the Builth 

 district, but in the Long Mountain there is an entire absence of 

 limestones, and the sediments range from thin papery shales on the 

 one hand to hard, sandy, flaggy mudstones on the other. 



(2) Northern Area of the Long Mountain. 



(a) Trefnant-to-Middletown Stream Section. — The 

 most complete section on the north side of the Long Mountain 

 occurs in the brook which flows from Upper Trefnant on the south 

 to near Middletown Railway-station on the north. 



As we ascend the section from that station, greyish-black shales 

 containing Cyrtograptus Lundgreni, Monograptus Flemingii var. §,. 

 etc., occur in the stream-course where it runs parallel with the 

 railway (1 in the map, PI. XXVI), thus fixing the stratigraphical 

 position of the beds as the highest zone of the Wenlock Shales. 

 There are few exposures until (2) is reached, where a small tributary 

 brook from Glyn Common enters the main stream. Here the beds 

 consist of harder and more flaggy shales, slightly calcareous, giving 

 rise to a well-marked feature. They contain various brachiopods 

 and trilobites, but relatively few graptolites : such as occur being 

 assignable almost exclusively to M. dubius and M. vulgaris. Similar 

 shales are exposed for a considerable distance up stream, but become 

 increasingly massive and flaggy, and occasionally show concretionary 

 structure (3). M. vulgaris is by far the most abundant graptolite, 

 but species of Orthoceras and trilobites, etc., are numerous. The 

 graptolites are, on the whole, badly preserved, being rarely seen 

 in true profile, so that it is difficult to be certain of all identifica- 

 tions, and other species may occur besides M. vulgaris. 



At (4) the main stream follows the general strike of the beds 

 for some distance, but the section is continued along the tributary 

 stream which rises near Upper Trefnant. The shales with M. vulgaris 

 are shown along the stream for some distance farther. The direction 

 of dip of the beds varies from south 60° east to south 20° west, but 

 the amount of dip is small and fairly constant. 



At (5) the graptolitic fauna becomes more varied, and the beds 

 have yielded M. bohemicus, M. Nilssoni, M. uncinatus var. orbatus 

 nov., M. varians, M. dubius, and M. vulgaris var. a. The beds 

 consist of dark grey shales which weather to a light brown. The 

 main stream here bends abruptly to the left, and follows the strike 

 of the beds ; but a small tributary brook joins it on the right, and 



