386 F. J. North — Silurian Inlier near Cardiff. 



Section of the Borehole. 



Site of boring, the southern end of Helen Street, 1 Boath ; 6 in. ordnance 

 map, Glam. 43, S.E. Height of surface above O.D. 38 feet. 



Superficial deposits. 



Soil and terrace gravels 



Trias. 



Bed and green Keuper marls with gypsum 

 Conglomeratic series .... 

 Old Bed Sandstone. 



Purple micaceous sandstone, mottled marls 

 with cornstones ; grit band at the base . 132 474 



Silurian {Ludlow). 



Bed and grey fossiliferous mudstones, with 



sandstones and very thin calcareous bands . 152 7 626 7 



The lower part of the Old Red Sandstone Series includes red 

 micaceous mudstones in which Lingula and crustacean tracks occur. 

 Beneath the mudstones there are about 6 feet of a hard grit containing 

 fragments of the underlying rock. Mr. T. C. Cantrill, who has 

 kindly examined some of the specimens, compares the crustacean 

 tracks to those described by G. E. Roberts from the lower part of the 

 Old Red Sandstone at Bouldon Quarry, near Ludlow. 2 



The Silurian rocks yielded a rich fauna, including many Mollusca, 

 Brachiopoda, and Trilobita. The beds are regarded as of Ludlow age, 

 for although many of the forms present are such as occur in both 

 Wenlock and Ludlow beds of other localities, certain species, e.g., 

 Homalonotus hnightii, Konig, Murchhonia lloydi, J. de C. Sowerby, 

 Stropheodonta filosa (J. de C. Sowerby), Chonetes striatella (Dalman), 

 and Cyclonema turbinatum, Sollas, are especially characteristic of the 

 local Ludlow beds. 



The general direction of the concealed boundary between the 

 Silurian and the Old Red Sandstone can now be inferred from the 

 following evidence : — 



1. Trias was found to rest upon Silurian strata in a boring by the 

 side of the Rhymney Railway, about 500 yards south of the main 

 Silurian outcrop, 3 at a spot indicated by E on the accompanying map. 



2. In the Cardiff Brick Company's pit at Maendy (F on the map), 

 Trias rests upon Old Red Sandstone. 4 



3. In the Helen Street boring, now described, Trias rests upon 

 Old Red Sandstone. 



The junction therefore occurs north of a line from Maendy to 

 Helen Street, and south of the spot E ; its approximate position is 

 indicated on the map. 



If the Trias were removed, the Cardiff Silurian inlier would 

 appear as an oval mass, about three miles from east to west and about 

 two miles from north to south. Structurally it is an anticlinal fold, 

 trending in an east to west direction, i.e. in the direction of the 



1 Indicated by D on the accompanying Map (Plate XIII). 



2 G. E. Roberts, Q.J.G.S., vol. xix, pp. 233-5, 1863. 



3 The Country around Cardiff (Mem. Geol. Surv.), 1912, p. 105. 



* Ibid., p. 17 



