14 PROF. E. J. GARWOOD A1N T D MISS E. GOODYEAR [vol. lxxiv, 



Yat Hill in the floor of the old quarry immediately north of Dolyhir 

 Station, where the limestone has been worked down to the surface 

 of the Pre-Cambrian rocks (Quarry Gr). These basement-beds should 

 therefore crop out continuously between these two points through 

 the middle of Yat Wood, as indicated on the map, but no exposures 

 can now be seen. On the south side of the railway the junction 

 may again be observed, beside the path leading from the level- 

 crossing to Strinds Cottage (Quarry B). Here, again, although 

 the beds can still be traced, they have been almost entirely quarried 

 away. The best exposure showing this unconformity, however, 

 occurs at the present time in the new Dolyhir Quarry (Quarry D). 

 This quarry is divided into two portions by a vertical north- and- 

 south fault, which throws the limestone down about 50 feet to the 

 west. On the upthrow, or eastern side of the fault, the Pre- 

 Cambrian rocks are exposed in a small cliff close to the tram-line. 

 This exposure shows an admirable section of the basement-beds of 

 the limestone resting unconformably on the Pre-Cambrian rocks, 

 which here consist chiefly of fine grit, but include also one of the 

 bands of coarse grey conglomerate described above. The limestone 

 rests partly on the grit and partly on the conglomerate, the latter 

 being well seen at the southern end of the exposure. 



The basement conglomerate here reaches a thickness of -j feet, 

 and consists of angular and subangular fragments of Pre-Cambrian 

 grit, ranging in size from small grains to pieces 4 inches in diameter. 

 It contains, in addition, a few rounded pebbles of quartz derived 

 from the underlying conglomerate (PL I). The angular frag- 

 ments of grit appear to have been derived by subaerial weathering 

 directly from the Pre-Cambrian rocks, since many of them show 

 little or no sign of attrition bv wave-action. This fact would 

 appear to point to the conclusion that the Pre-Cambrian rocks had 

 already been shattered and brecciated in Pre-Silurian times. 



The section is complicated in places by the presence of an irregular 

 line of thrusting, which occurs near the junction of the two forma- 

 tions. Along this line the surface of the Pre-Cambrian rocks has 

 been torn and broken, giving rise to a crush-conglomerate composed 

 of quartz-pebbles embedded in a fine green rock-paste. The beds 

 along this line are impregnated with' copper salts, which include 

 numerous small crystals of copper pyrites and specks of chessylite 

 and malachite. The basement-beds of the limestone contain 

 numerous tabular colonies of JFavosites measuring several inches 

 across, which lie in the position of growth among the fragments of 

 Pre-Cambrian grit, and mark the bedding-planes. The whole 

 deposit is cemented by a coarse matrix, composed of crinoid 

 ossicles embedded in secondary calcite. The conglomerate also 

 shows evidence of crushing, the planes of movement being again 

 impregnated with copper salts. This basement conglomerate dips 

 at about 12° eastwards, and is immediately overlain by white 

 crystalline limestone containing abundant remains of Solcnopora. 



The limestone. — The main mass of the limestone is well 



