F. R. Cou']3er Reed — Blind Trilohites. 493 



by a blue line. Thus a normal fault completely proved would be 

 represented by four lines, the order in which the lines occur 

 indicating the direction, and their distance apart the angle of the 

 hade. A further difficulty remains, however ; for the plan-position 

 of a fault encountered in any vein in working up to it from the east 

 would not be the same as the plan-position of the same fault in the 

 same vein if worked up to from the west, owing to the hade. lu 

 a fault of 100 yards the discrepancy would amount to 35 or 40 yards, 

 and it becomes necessary to record also from which side the fault 

 was proved, which is not always easily ascertained in old workings. 

 The coloured lines referred to are used on the B-inch maps only ; on 

 the l-inch maps the underground faults are all shown by yellow 

 lines to avoid undue complication. 



The glacial deposits are mapped simultaneously with the solid 

 geology, and are shown on the edition of the map for superficial 

 geology. With the exception of the admirable work of Professor 

 Edgeworth David, and observations by the late Eev. W. S. 

 Symonds, they have not attracted so much attention as they deserve, 

 for South Wales formed a small independent centre of glaciation. 

 and exhibits phenomena of great interest. The greater water- 

 partings of the present day formed the ice-partings of the Glacial 

 Period, and the principal valleys gave the route to the ice-flow. 

 Thus a great mass of drift was transported from Brecknock round 

 the north-east corner of the Coalfield down the Usk Yalley as far as 

 the depression occupied by the Usk Branch Eailway. Another part 

 was pushed over a minor water-parting into the Khymney Valley, 

 but principally escaped along the Taff Valley, traversing the entire 

 Coalfield and emerging by the ravine at Walnut Tree ; while 

 a third portion flowed south-westwards along the Neath and other 

 valleys towards Swansea Bay. The drift consists in part of coarse 

 gravels or fine gravel and sand, and forms characteristic mounds or 

 ridges between which are inclosed innumerable water - logged 

 hollows, or meres. Nearer to its source, however, it becomes an 

 extremely tough boulder-clay packed with glaciated boulders. The 

 composition of the deposit, the direction of the longer axes of the 

 mounds, and, lastly, a large number of striations on rock-in-place 

 combine in determining the directions assigned to the ice-flow. The 

 southern limit to which the ice reached is no less clearly marked 

 than its birthplace, for the gravels get finer and thinner, and 

 eventually die away, sometimes before reaching the shores of the 

 Bristol Channel. 



III. — Blind Trilobites. 

 By F. E. CowPEU Reed, M.A., F.G.S. 



{Continued from the October Number, p. 447.) 



(2) Order Opisthoparia. 



WE now come to the second order of the Trilobita, which Beecher 

 (loc. cit.) calls the Opisthoparia. In common with the third 

 order, the Proparia, compound paired eyes are usually present, and 

 these are 'invariably situated on the free cheeks, which form part of 

 the dorsal surface of the head-shield. 



