12 



Green — Carboniferous Rocks of N. Wales. 



of them, could be recognised among the Lower Carboniferous Eocks 

 of North Wales, and I came to the conclusion that the Millstone Grit 

 is there feebly represented, and that the greater part of these beds 

 is more likely to be the equivalent of a portion of the Yoredale 

 Group. 



I hope I shall not be thought guilty of presumption if I seem to 

 differ from the eminent geologists who have called the whole of the 

 rocks under consideration Millstone Grit. These authors seem to me 

 to have used that name in a wide and somewhat vague sense, mean- 

 ing by it, any sandstones which are found between the Coal Measures 

 and the Mountain Limestone ; and they would, I have no doubt, 

 have classed as Millstone Grit, the beds which, under the scheme 

 just given, form the two upper divisions of the Yoredale Eocks. The 

 difference is thus, after all, one of words ^ only ; and all I can claim 

 is to have endeavoured to give a rather more detailed account of 

 these rocks, and to fix more definitely their place in the geological 

 scale ; with what success further research must determine. 



The rocks I was able to examine cover a belt of coimtry, about two 

 miles broad, between the Flintshire Coal Field and the Mountain 

 Limestone of the Eglwysegle Eocks, and consist of sandstones parted 

 by beds of shale. 



A good section of the lowest beds is seen near Tan-y-Castell, one 

 mile and a quarter north-east of Llangollen. It is figured in the 

 woodcut below, and shews the following beds, in ascending order. 



Section No. 1. 

 f 1 . Solid grey limestone. 



I 2, Limestone, thinly bedded, somewhat earthy. Corals and shells in 

 ^ plenty. 



Flaggy limestone, sandy, and holding here and there many small 

 quartz pebbles. Band of encrinital limestone at the top. 

 4. Very closely-grained hard sandstone, highly quartzose, with small 

 white quartz pebbles, most likely a calcareous cement, weathers 

 into a honey-combed form. 

 '. Finely grained, softish sandstone, here and there full of small, white 

 quartz pebbles. 



A. Light sandy drift, pebbles of slate and igneous rocks. 



B. Boulders of various igneous rocks. 



A. B. 



Mountain 



Limestone. 



1 2 3 4 



Section across the Eglwysegle Rocks, near Tan-y-Castell. 



Though not immediately connected with our subject, the gradual 

 passage upwards from the limestone into the sandstones is worth 



^ A recollection of this fact would have saved much needless controversy as to which 

 of the many schemes for the subdivision of the Lower Carboniferous Eocks is most 

 natural. All must be alike artificial and matter of convenience, for the simple reason 

 that, with local exceptions, the Carboniferous Group is conformable from top to bottom, 

 and so no lines of subdivision have been marked out in it by nature. One scheme 

 may be more handy for certain districts, or more suited to individual taste, than 

 another, but this does not make it more natural. 



