108 Geology of Wiltshire. 
The hills composed of chalk are so porous, owing to the loose 
texture and numerous crevices of the rock, that the rain falling 
upon them sinks into the mass, and rarely runs upon the surface. 
Hence its general dryness, and the number of dry combes or hollows 
by which its surface is scored, and in which water seldom flows. 
The mass of chalk is, however, very retentive of moisture, and acts, 
probably, like a great sponge, holding it long in its pores and cre- 
vices, and giving it out by degrees in the springs which gush out 
at low points, to which’ the uneven surface of some bed of the un- 
derlying clayey gault may direct its flow. And hence the river 
waters of the chalk district, being thus slowly filtered, are usually 
very clear, fine, and equable. It is, also, acireumstance worthy of 
remark, that the rivers which drain all the large area of the chalk 
district, do not flow in the direction which would seem most natural, 
namely, that of the great vales which open out from it towards tie 
west, but through narrow channels that cut across the whole breadth 
of the chalk platform from N. to 8., or from W. to E., and which, 
probably, had their origin in cracks or fissures broken through 
this tabular mass of chalk strata at the time it was elevated to its 
present high level above the sea in whose depths it was deposited. 
Such are the valleys of the Kennet in the North, and of the 
Bourne, Avon, Wily, Nadder, &c., in the South. The same fact 
is observable in the chalk districts of the counties of Surrey, Kent, 
and Sussex. Each of these valley cracks is, probably, coincident 
with an anticlinal axis, or saddle-back arrangement, of one of the 
secondary, or transverse curvatures of the elevated chalk strata. 
The Lower Green sand in Wiltshire rather deserves the name of 
Tron or Ferruginous sand, since it generally contains so large a pro- 
portion of that metal, as to render many of its beds workable as 
smelting ores. This has long been known, and, indeed, traces are 
frequent of old smelting furnaces within the Lower Green sand 
area, from Seend to Bromham and Sandy-Lane. But the process 
could only be carried on successfully, so long as the neighbouring 
forest of Pewsham supplied the necessary fuel. That consumed, 
the ore, however rich, became valueless, until the railway pierced 
this district, and by offering facilities for the conveyance either of 
