THE PATLI DUN. 73 



is much reduced on account of the direction of the section being 

 diagonal, and not at right angles, to the strike. 



The nummulitics in the river-bed show a certain amount of purple 

 shale associated with them, which in some instances is very difficult 

 to distinguish from the purple slate series to the north. Generally, 

 however, it may be laid down that the beds near the nummulitic 

 calcareous layers are less hardened and slaty, more of a shale in fact, 

 and they contain a sufficient amount of lime to effervesce slightly 

 with acid. Among them two or more bands of grey nodular impure 

 earthy limestone appear containing nummulites. In certain places 

 on the ridges the nummulitics have also a sandy element, which 

 occasionally weathers into a soft brownish sandy rock, very friable, 

 in which casts of foraminifera and bivalves can be seen. It may be 

 remarked here that generally on the ridges, away from the influence 

 of the river, the strata have become softer, and can be more readily 

 and sharply divided off from the purple slate series. This is not 

 altogether due to the weathering of the rock in this more exposed 

 position, but also to the selective action of the river, which has cut 

 away all the softer portions of the beds, leaving only standing up in 

 small cliffs on the visible section those portions which were sufficient- 

 ly hard to offer a certain amount of resistance to the river action. 

 River sections in very many cases give a better index of the structure 

 (dip, sequence, Sec.) of the rocks than of the finer differences of com- 

 position : the laving water cuts and moulds every rock to a much 

 more uniform appearance than the slower action of sub-aerial agencies. 



On the north edge of the nummulite-bearing calcareous layers, there 

 is a doubtful zone of purple shales and purple grits, which sometimes 

 seem to merge into slates ; after which there come grey slates with 

 wavy bedding, almost vertical and passing upwards into undoubted 

 thorough-going slates of purple and greenish colour, and these in 

 turn finally pass into the volcanic breccia. The grey and purple 

 slates and breccia are almost vertical, and somewhat folded among 

 themselves, no doubt, but it is difficult to say how exactly. The 

 latter half of the purple slates and the southern half of the breccia 



( 131 ) 



