152 ROCK STRATA 



Tliese. rocks differ a little in colour ; but in 

 other respects they might all be from the same 

 place, though they are probably separated by 

 intervening strata, as well as distance, horizon- 

 tally. They consist of particles of sand weakly 

 agglutinated, and coloured with an indefinitely 

 fine coating of oxide of iron. They are easily 

 reduced into sand, which, being heated to red- 

 ness, loses 1 or 2 per cent, of weight from the 

 inherent moisture. After this, if the residue be 

 treated with dilute, pure, colourless muriatic 

 acid, about 1 per cent, of oxide of iron is ab- 

 stracted, and there remains a fine while sand, 

 shewing, that the iron only coats the surface of 

 the sand, though it gives it the appearance of a 

 body of the red oxide of that metal. 



6. Ardwick Limestone. 

 2.71 Specific Gravity.* 



A bed of limestone is found, at a considerable 

 depth imder the ground, to the east of the town. 



* The specific gravity of pure white marble, and all or most of 

 the compact mountain limestones, is nearly 2.7- — ^The specific 

 gravity of the true magnesian limestone, from Yorkshire, is 

 usually much less; being only about 2.1 or 2.2, agreeing with 

 that of soft chalk nearly : the hard chalk of Lincolnshire, I find 

 nearly 2.6 specific gravity. All these kinds, of low specific gra- 

 vity, however, it must be observed, are porous, and imbibe less or 

 more of water. 



