Vol. 62.] CARBONIFEROUS ROCKS AT RUSH. 283 



They consist of black shales, sometimes calcareous, interstratified 

 with thin flaggy bands of sandy limestone containing quartz-pebbles 

 and rock-fragments, for the most part of small size. Cleavage is 

 beginning to die out, and is weak, except in the basal beds. 



Fossils have been collected from various horizons in the Rush 

 Slates, and those sufficiently well preserved to be identified are 

 recorded on a later page (p. 295). The lowest 200 feet have not yet 

 yielded fossils ; but they may be provisionally grouped in the same 

 zone as the overlying beds. The beds between II 1 b and R 8 a seem 

 to belong to the 'Upper Zap7ire?itis-Zone (Z2-y)' of Dr. Yaughan, 

 their upper portion (R 6 b to R 8 a) containing a typical ' y ' fauna. 

 From R8<x into the overlying conglomerates the beds are assigned to 

 the ' Main Si/rinr/othyris-Zone ' (C). The typical fossil of all these 

 beds is Zaphrentis cf. Phillipsi, Edw. & H. 



Before passing on to the description of the Rush Conglomerates, 

 the more striking effects of compression on the Rush Slates ought 

 to be noticed. The shallow basin of Lower Slates on the southern 

 shore is crossed, as already stated, by cleavage striking nearly east- 

 and-west, with the result that over the larger part of the exposures 

 there is a marked discordance between bedding- and cleavage-strikes. 

 Owing to the fact that the shales have yielded to compression more 

 than the limestone-bands, the latter have usually been crumpled at 

 right-angles to the cleavage-strike, in order to reduce their horizontal 

 extent. Fig. 4 (p. 284-) shows the lobe-like extensions which this 

 oblique direction of compression produces on the outcrop. It will be 

 noticed that the detailed folding of the bed is quite discordant with 

 the general dip. Occasionally the folding is so pronounced as to 

 produce a slight inversion of the bed, and the concave surfaces are 

 puckered into minute corrugations. The crumpling dies out in the 

 slates in a very few feet vertically above and below the crumpled 

 bed. From a measurement, necessarily rough, of the surface of the 

 limestone taken across the folding, it appears that 5 feet of lime- 

 stone corresponds to about 3 feet of slate ; and, as this would 

 represent the minimum compression, we may infer that the slates 

 have been reduced by nearly one-half their original extent across 

 the cleavage-strike. One effect of the compression on the slates is 

 that their thickness will be found to vary if measured in different 

 directions, being least where the bedding-strike coincides with 

 the cleavage -strike, and greatest where the two strikes are at right- 

 angles. 1 



The augen-like nodules of impure limestone which are common in 



1 The peculiar stratigrapbical features of these limestone-bands, although 

 occurring on a small scale and in other ways exhibiting considerable differences, 

 are recommended, to the notice of geologists interested in the much-debated 

 structures of limestone-knolls. Substitute for the thin bands in the Rush Slates 

 a massive but flexible limestone lying between masses of more compressible 

 strata, and for the overlying Rush Conglomerates substitute a thick mass of 

 Millstone Grit, and subject the whole to powerful compression: then one may 

 expect as a result, that the surface of the limestone will be thrown into folds 

 which would be locally unconformable to the adjacent strata, and that about the 

 junction some brecciation may be looked for. 



