ROCKS OF THE FOREST OF DEAN. 215 



3. The Millstone Grit overlies the Carboniferous Limestone at an 

 angle of 45° ; and the lower beds have some features which are not 

 common to the upper. Taking first the former, we find (1) that 

 they are built up of fine-grained sandstones together with reddish- 

 coloured flagstones. (2) There are casts of shells and of other forms of 

 life. (3) Remains of a Lepidodenclron allied to L. Griffithii (Brongn.), 

 are very numerous. The sandstones are composed almost entirely of 

 quartz, with a very small quantity of mica and felspar. Coming to 

 the upper beds, they consist of sandstones, the grains of which 

 are well waterworn ; but no signs of life of any kind have as yet 

 been found in them. In some of the beds " vein-stone " quartz 

 pebbles occur ; and these when compared with those in the Old 

 Red Conglomerate seem to be identical, except that they are more 

 rounded. One bed, at Sudley, is so full of these pebbles that when 

 first discovered it was taken for the Old Red faulted up *, but on 

 examination was found to be regularly interstratified with the other 

 beds, about the determination of which there was no question or 

 doubt. The following is an analysis giving the chemical com- 

 position of a typical representative of the Drybrook Millstone Grit ; 

 and for the sake of comparison an analysis of the same formation 

 from Brandon Hill, Bristol, is also given. 



Forest of Dean. Bristol. 



Silica • 98-06 97-80 



Alumina -30 -47 



Oxides of iron *50 '80 



Lime -33 -44 



Carbonic acid *30 '39 



Alkalies trace. 



Loss on ignition (carbon) *20 "17 



Moisture -22 



99-69 100-29 



Reviewing the evidence of sequence, the Old Red Conglomerate, 

 the many- coloured sandy beds and calcareous shales which imme- 

 diately follow, and the Millstone Grit, all seem to have been 

 formed by materials derived from the same source, namely ancient 

 granitic rocks ; but the sediment and pebbles have been subjected to 

 various degrees of mechanical attrition : the evidence of this is fur- 

 nished by the occurrence of the same minerals in these rocks, and 

 especially by the "vein-stone" quartz pebbles. There is no uncon- 

 formability ; and therefore there is no reason for supposing that one 

 set of beds can have originated from the denudation of the other : 

 this being so, there is no alternative but to accept the view which 

 I have expressed. 



As to the correlation, I will only say that the sandy beds and 

 calcareous shales, between the Old Red Conglomerate and Lower 

 Limestone Shales, have no Old Red affinities, and occupy the position 

 of the Calciferous Sandstone group in Scotland ; the lower beds of 



* I hare since come across the same bed in another place regularly interstra- 

 tified with the Millstone Grit. 



