

ECONOMIC ASPECTS OF GEOLOGICAL STRUCTURE 345 
Foundations.—Engineers and architects are necessarily 
called upon to consider the nature of the foundations on 
which it is proposed to build heavy structures. Trial holes 
will show the nature of the materials, but they do not always 
disclose the geological structure, and in the case of very 
heavy buildings it is absolutely essential that the latter should 
be carefully ascertained. For, however solid and unyielding 
the substratum may seem to be, calculations as to its stability 
are liable to error if its relations to the immediately subjacent 
rocks be not taken into account. For example, a firm 
massive sandstone may be underlaid by some impervious 
slippery clay, which, should the strata have a decided dip, 
may yield to the great pressure of a heavy superstructure, 
and cause the rock-foundation to slide forward along the 
plane of bedding. Unconsolidated materials often make bad 
foundations, but tough, homogeneous clay, if of sufficient thick- 
ness, is usually reliable. Alluvial or superficial deposits of 
every kind, however, are as a rule unsatisfactory, and, in 
the case of important structures, usually require special treat- 
ment, involving often costly excavation, the driving of strong 
and closely set piles, and the formation of an artificial founda- 
tion of concrete. Although tough clays, such as the boulder- 
clay of the Scottish lowlands, usually form reliable foundations, 
they nevertheless are sometimes untrustworthy. Not infre- 
quently they contain layers and beds of gravel and sand that 
carry water, which, when it escapes at the surface, tends to 
undermine the overlying mass, and thus in time causes the 
ground to subside. Before any heavy building is raised upon 
till, therefore, it is necessary to ascertain by means of boring 
whether any water-bearing beds be present. The river- 
valleys of Central Scotland afford many examples of the 
relative instability of boulder-clay, when that deposit rests 
upon an inclined surface of rock. In the valley of the Esk, 
for example, almost every house and wall built upon the 
slopes overlooking the valley-bottom afford evidence in their 
cracked masonry of a slow and interrupted but nevertheless 
continuous slipping of the foundations, The cause is obvious: 
the boulder-clay, which has a coarse, rubbly bottom, rests 
upon an inclined surface of sandstones, shales, etc., from 
which water escapes and percolates through the stony and 
