' 24 
ourselves principally to the study or how these rocks and mineral veins were 
formed. 
It is needless to go back to the time when the earth existed in the form of gas, 
but it will serve our purpose if we commence with the formation of the oldest- 
sedimentary rocks of which we know anything. The first great principle to fix in 
one’s mind and never lose sight of is that during all geological periods things have 
been going on much as they do to-day, with only the difference that the physical 
forces at work are gradually losing their intensity, and the earth is now settling 
down as a man does as he advances in years. In the early days of the earth’s 
history, earthquakes were probably more violent and frequent than to-day; there 
was greater volcanic activity, and probably a much greater rainfall, with a cor¬ 
respondingly increased denuding action. 
This difference may be accounted for by the fact that the earth’s internal heat 
was far greater then than it is now, which would cause greater evaporation from 
the earth itself; but as it was probably slightly further from the sun than it is 
now, the upper atmosphere would be correspondingly cooler, which would cause a 
rapid condensation of the aqueous vapour as it rose, causing it to fall as rain or 
snow. 
The solid crust of the earth was also much thinner, which would render it 
liable to be frequently broken by the earth strains, and up the fissures so caused 
molten matter would be forced, with thermal waters at enormous temperatures 
with mineral matter in solution. The shape of the land surface, too, would prob¬ 
ably have been very different, the hills attaining much greater elevations, for after 
the first consolidation of the earth’s crust it would have been broken and thrown 
up on end many times from the contraction of the inner mass as it cooled, until it 
eventually attained too great a thickness to be affected in such a marked degree. 
Now, supposing we start at this period, when the oldest known sedimentary 
rocks were being formed at the bottom of the ocean (bedded rocks are formed 
in this manner). The heavy rains falling upon the hills would rush down their 
steep faces, breaking away the rocks, and carrying them down in the form of 
mud, sand, gravel, and boulders towards the sea, where they are sorted by gravita¬ 
tion, the coarser nearer the shore, and the finer, which, would be held'longer in 
suspension, further out; this latter would form mud beds in the deep water, whilst 
the sands, gravels, and boulders would be deposited near the shore. 
In course of time bed upon bed would be built up in this manner, when, with 
pressure and heat, they would be altered, the mud into clay, shale, slate, schists 
&c., and the sands, gravels, and boulders, by the passage of thermal waters, into 
sandstones, quartzites, and conglomerates. 
But these beds liave not remained in the horizontal position in which they 
were deposited, but were, whilst they were yet flexible, laterally compressed by the 
shrinking of the earth’s crust, which folded and contorted them, more beds bein<>- 
deposited unconformably upon these, and the whole gradually compressed and 
consolidated. 
In the course of time these old sea bottoms were gradually elevated, and 
became land surfaces, whilst other tracts which had been land gradually subsided 
and were covered by the sea. 
The more modern of the sedimentary rocks were deposited in a similar 
manner, but have been generally built up more slowly, and have not been altered 
or contorted so much, 
When the old rocks, in time, became quite solid aud would no longer adapt 
themselves to the contraction of the internal mass they had to break, and up the 
fissures thus formed quantities of molten rock were obtruded, often in such quanti¬ 
ties and at such a high temperature as to melt the sedimentary rocks on either 
side for a considerable distance; but as these rocks were melted under great 
