232 



REPORT — 1882. 



Two typical specimens of Millstoue Grrit were selected from Pertyrcb, 

 South Wales ; one a coarse variety and the other with grains averaging 

 about O'OIO of an inch in diameter. There is a slight lithological differ- 

 ence when compared with the same formation around Bristol, and 

 chemically it is a little more argillaceous, as shown by the following 

 analysis : — 



Silica 96-63 



Alumina . 



Oxide of iron 



Lime 



Carbonic acid 



Carbon . 



Moisture . 



Alkalies not estimated 



]-15 

 •70 

 •55 

 •20 

 ■30 

 •10 



99G3 



The specific gravity may be taken at 2^57, the volume of water 

 capable of being absorbed by a cubic foot of the rock 0-290 of a gallon, 

 or 23,000,000 gallons to a square mile 3 feet thick. 



The specimens of Millstone Grit from Sheffield resemble lithologically 

 those of South Wales, though a person acquainted with the formations 

 in the two districts would probably be able to distinguish between them. 

 The grains composing the grit in the neighbourhood of ShefiBeld, 

 according to Dr. Sorby, 'are, on the whole, extremely angular.' ' The 

 specific gravity averages 2'59, and the volume of water absorbed per 

 cubic foot of rock is 0-504 of a gallon, equal to 42,000,000 gallons to a 

 square mile 3 feet thick. 



Two samples of Millstone Grit were collected from the Forest of 

 Dean coalfield ; one from the southern outcrop and the other from the 

 northern. We here get a very different lithological character when com- 

 pared with the same formation at Bristol and in South Wales, and a 

 very much larger volume of water is absorbed. The rock resembles a 

 Trias sandstone more than the Millstone Grit, but in chemical com- 

 position there is practically little difference, as is shown by the following 

 analysis of a specimen from Drybrook : — 



Silica 98-06 



Alumina -30 



Oxide of iron , . -50 



Lime -33 



Carbon -20 



Carbonic acid -30 



Alkalies Trace 



99^69 



The specific gravity of tlie Forest of Dean Millstone Grit averages 

 2-63, the volume of water absorbed by a cubic foot of the rock is 0854 of 

 a gallon, or 71,000,000 gallons to a square mile 3 feet thick. Though I 

 have given the average volume of water absorbed, it will be seen, on 

 reference to the tabulated list, that the results obtained from the two 

 specimens examined differ considerably. In the case of the one absorb- 

 ing the greatest quantity of water, the grains composing it were but 

 slightly cohesive, but in the other case the rock was of a more compact 

 character. 



Quarterly Journal Geol. Society, 188, page 64. 



