486 Rev, J. F. Blake — Precambrian Geology, 



"The grit is here seen to be unconformable, but the evidence is 

 scarcely satisfactory or conclusive. If it had been conformable, the 

 motion which is indicated by the slickensides might easily have 

 produced the amount of unconformity apparent." Dr. Callaway 

 says just the same, "The grit appears to have been a little squeezed 

 into the slate, so that here and there a trivial unconformity is 

 apparent." According to his section, which, by the way, is not 

 what is seen at any one spot, but is made up from several exposures, 

 all the rocks have the same dip, so there is an apparent contormity. 

 It is not easy to see how an apparent conformity combined with 

 an apparent unconformity can " absolutely disprove " the reality of 

 either one or the other. My deduction is that the evidence here is 

 not conclusive. But other evidence is, or at least is put forward as 

 such, but it is not tackled by the critic. 



With regard to the conglomerates and grits found on the "Volcanic 

 Hills" being superficial or otherwise, it will be well to state what 

 I regard as evidence of a rock being superficial. When the boundary 

 of a rock is of an irregular kind, like the outline of a map, and is 

 surrounded on all sides by other rocks which have a more definite 

 strike, particularly if its boundary is nearly horizontal and follows 

 approximately the contours of the present valleys, 1 take it to be 

 superficial. In proportion, of course, as these features are wanting, 

 the evidence is less satisfactory. They are all fulfilled by the grit 

 on Caidington Hill. I am not at all sure that Dr. Callaway has seen 

 this patch, for the rock which composes it is not like the Woodgate 

 quarry rock. As to Charlton Hill, Dr. Callaway gives a section, 

 but this section is not seen, but inferred. The map I give is what 

 is actually seen, and this proves the inference to be wrong. I am 

 sorry to say that this map, on p. 410, is so small that it requires 

 a lens to examine it. Small cii'cles on it indicate exposures, and 

 the boundary lines are drawn to include exposures of similar rock. 

 Let Dr. Callaway show that this is wrong. It is drawn carefully to 

 scale on the One-inch Ordnance Map, and there cannot be the slightest 

 difficulty in knowing the exact spots intended. The same remark 

 will apply to the two patches of igneous rock " near the south- 

 eastern end of the Wrekin," as to which he says he is in the dark; 

 a lens will show hinj exactly yvXi^re they are on the One-inch Map. 



On the crystalline rock of the Ercal, it will be sufficient to compare 

 the two following passages : " Prof. Bonney said the rhyolite was 

 clearly intrusive in the granitoid series" (Q.J.G.S. 1879, p. 6G9). 

 "He" (Prof. Bonney) "cannot find any distinct proof of the intrusion 

 of the felsite in the granite" (Q.J.G.S. 1891, p. 118). 



Finally, with regard to the difference in strike of the rocks of the 

 Volcanic Hills and the Longraynd rocks, it does not appear to me 

 to be of any great consequence, and in any case does not prove the 

 former to be older. Many of the rocks in the former have no strike 

 at all, and the strikes noticed are very variable. Volcanic rocks, even 

 when clastic, are apt originally to have a quaquaversal dip, and more 

 than all, the rocks on the east side of the great fault have had to 

 bear the brunt of all the earth-movements, and they have yielded to 

 them in no simple way. 



