1882.J 
Geological Faults . 89 
contrasted with that on the other, Mr. Wallace, in his 
interesting work on “ The Laws which Regulate the Depo- 
sition of Lead Ore in Veins,” says that in some cases the 
strata had been subjected to much tension before it was 
fraCtured is a conclusion “which hardly admits of a doubt.” 
Referring to this I have elsewhere observed — “ It is obvious 
that expansion from growth would be likely to have such an 
effeCt upon strata of a hard, unyielding nature, which would 
ultimately crack. Often these cracks (or veins), instead of 
rising vertically from below, pursue a considerable angle, 
and sometimes two cracks at no great distance from each 
other will take opposite directions, so that there is reason 
to believe that they * hade ’ into each other and become one 
crack at no great depth from the surface. When this occurs, 
the intermediate strata between the two cracks, which are 
of a V or wedge form, is usually ‘ thrown down.’ The 
cracks having widened into fissures by the Earth’s expan- 
sion, the intermediate strata have slowly ‘ subsided.’ ” ( See 
“ The Constitution of the Earth,” p. 362.) Water-courses 
have nearly in all instances originated in cracks, and although 
denudation has often played an important part in the existing 
conformation of valleys, they are mainly the result of an 
exceedingly slow separation of the two sides of what was 
once a crack. {See the article on 4 Scepticism in Geology,’ in 
“ The Journal of Science ” for June, 1881.) In making the 
foundations of the unfortunate Tay Bridge, evidence was 
discovered of what I take to be three distinct cracks, the 
result of the Earth’s expansion. First, the primitive rock 
has split at a very remote period in geological history, and 
at the present time we find the two sides separated to the 
extent of a mile and a half. A gorge, 500 yards wide, was 
in the first instance slowly formed, which was filled up by 
sedimentary deposits, including red sandstone, which is now 
found immediately below the superficial clay. A second 
crack then took place, by which the red sandstone deposit 
was separated from the trap on the north side of the river. 
Over an immense period of time, this slowly widened out to 
the extent of more than a mile, during which further sedi- 
mentary deposits were formed. Between the latter and the 
red sandstone a third crack next occurred, and this is now 
filled up by soft material, which gave the bridge contractors 
a great deal of trouble. It is now about 300 yards wide. 
In a history of the Tay Bridge, given by Mr. Albert Grothe, 
we are told that, “ after the fourteenth pier had been built, 
it was found that the rock suddenly dipped to a depth which 
was practically inaccessible, a discovery which led to a 
VOL. IV. (third series). h 
