ON THE CIRCULATION OF UNDERGROUND WATERS. 233 



Referring to the upland surface water from the Millstone Grit and 

 the non-calcareous portions of the coal measures, the Royal Com- 

 missioners on Rivers Pollution say in their sixth report,' ' Many of the 

 larsre manufacturing towns of Lancashire and Yorkshire are supplied 

 with water for potable and manufacturing purposes, by the storage in 

 vast reservoirs of the upland drainage from these formations. Being 

 but slightly absorbent they yield to the impounding rivulets and streams 

 a large proportion of the actual rainfall.' Though the Millstone Grit 

 may be but slightly absorbent in the localities named, yet this character 

 cannot be established as a rule by which one can be guided. In the case 

 of the Bristol coalfield it applies, but when we come to the Forest of 

 Dean we find the reverse to be the case. The Millstone Grit of the West 

 of England serves as a good illustration of the variability of rocks in 

 different localities, especially as regards the volume of water which is 

 capable of being stored in them. 



The next rocks examined were those of the Pennant Grit. It is 

 necessary here to lay stress upon the definite article, as ' the Pennant ' is 

 confined to the middle coal-measui-es of Bristol, and is also extensively 

 developed in the Somersetshire and South Wales coalfields. There are, 

 however, beds of grit in the lower coal-measures of Bristol, which are 

 lithologically true Pennants. The Pennant Grit is a compact blue rock, 

 made up of angular grains. The following is the mean analysis of seven 

 samples taken from my paper on the ' Composition of the Pennant Gi-it.' ^ 



Silica- ■ . 84-96 



Alumina ■i'34 



Oxide of iron ........ 4'55 



Lime 1-29 



Carbon 286 



Carbonic acid ........ 1'33 



Magnesia "05 



Water -58 



99-96 



The specific gravity of the Pennant averages 2'67, the water absorbed 

 by a cubic foot of the rock 0"150 of a gallon, and by a square mile 3 feet 

 thick 12,000,000 gallons. Speaking of deep wells in the Coal Meastires, 

 the Rivers Pollution Commissioners say : ^ The proportion of mineral 

 impurity present in the deep well-water is always large, but varies within 

 wide limits. The water was found to contain, ' as a rule, larger pro- 

 portions of organic elements (organic carbon and organic nitrogen) than 

 are met with in similar waters obtained from other strata,' the average 

 proportion being 0'153 parts per 100,000 parts, or 'lO? grains per gallon, 

 I quite endorse what the Commissioners say in respect to water from 

 the coal-measures generally, but where we have a great thickness of 

 rock, as in the case of the Pennant around Bristol and Swansea, I think 

 there may be an exception to what has been stated. Some years ago, 

 it was proposed to supply a portion of Bristol with water from the 

 Frampton Cottrell iron mines, which are in the Pennant, and the 

 analyses made of the water showed it to be of good quality. Considering 

 the quantity of water which this rock is capable of storing, the quality 



' Page 40, part 2. 



- Journal of the Cliem. Society, 1882, page 79. 



' Sixth Keport, page 91. 



