AND NEIGHBOURHOOD. 203 



sider their permeability by water, a property which 

 is generally noticed even by the most superficial 

 observers, and of great importance to the health 

 of a large town. 



Generally speaking the older strata, in the vici- 

 nity of Manchester, both carboniferous and new 

 red sandstone, are so thickly covered with Drift, 

 as to affect, only in a small degree, the hydrome- 

 trical state of the subsoils on which the town and 

 its suburbs are erected. The first named deposit 

 has scarcely any houses erected immediately upon 

 it ; and although the latter, in the lower parts of 

 the boroughs of Manchester and Salford, has a 

 considerable number resting upon it, they are very 

 few in comparison with those situated on the Drift. 

 Both formations present irregular surfaces, much 

 abraded, and rising up into the Drift, higher in 

 some situations than in others, as shown in the 

 section No. 2. It, no doubt, is owing to this 

 irregularity of surface that the lowest bed of gra- 

 vel and sand, hereafter described, is so uncertain 

 in its occurrence, and so very variable in its 

 thickness. The valley-gravel (No. 1) is never 

 found beyond a certain height above the level of 

 the Irwell ; and its exact height being once deter- 

 mined, it is very easy to trace through the town. 



