ROCK STRATA ABOUT MANCHESTER. 149 



rock, was 2.476 ; and that of the top rock was 

 2.256 only. The two last specific gravities, how- 

 ever, are not correct ; for the rock is so porous, 

 that it imbibes a considerable quantity of water, 

 by being immersed in that liquid for one or two 

 minutes. I found, that 924 grains of the middle 

 rock, when put into water, and thus submitted 

 to the vacuum of an air-pump, for ten minutes, 

 acquired 29 grains in weight : it was then dried 

 in a heat of 100°, when it lost 37 grains. After- 

 wards, being immersed in water for two minutes, 

 in order to find its specific gravity, it thereby 

 gained 27 grains in weight. Hence, it varied 4 

 per cent, in weight -, and this variation might, no 

 doubt, have been increased. An expert geologist 

 will, perhaps, account for the different specific 

 gravities of the rock at top and bottom, by ascrib- 

 ing it to pressure; but another, with greater plau- 

 sibility, may maintain, it is owing to the perco- 

 lation of water through the upper rock, carrying 

 the finer particles down into the pores of the 

 under rock, where they become fixed. Chemical 

 analysis is in favour of this last notion. The 

 bottom rock, I find, contains 4 or 6 per cent, 

 of carbonate of lime; the top contains none. 

 The chief constituent of both is sand, or sili- 

 ceous earth, combined, probably, with a little 

 alumine. The bottom rock, boiled in muriatic 

 acid, exhibits carbonate of lime, and a trace of 



