strata. We do not find it characterizing more modern rocks 
of analogous composition or condition. The existence of 
enormous deposits of rock, containing three-fifths silica and 
one-fifth alumina, exhibiting true slaty structure, is peculiar 
to this age of the geological scale. Such rocks are found 
extending over a large portion of the area of the land on 
the globe. The same composition and structure have been 
ascertained to exist in rocks of the same geological epoch right 
across both hemispheres, and well-nigh from pole to pole. 
29. The Carboniferous system is distinguished by the vast 
amount of carbon, in the form of coal, accumulated in its 
layers. The condition of things in regard to the growth of the 
vegetation whence the coal was derived was similar to the pre- 
sent. The sunshine and rain, winter and summer, river and 
lake, have all written their annals in the coal-beds. But there 
was a different distribution of land and water, and of terrestrial 
temperature, for we find traces of sub-tropical vegetation in 
the coal-sliales of the Arctic regions. Though coal has been 
formed both before and after the carboniferous and oolitic 
epochs, yet in the former was its principal development. Look- 
ing at the enormous development at this epoch of forest and 
swamp, composed of nearly identical vegetation in all parts of 
the world, we have only to remark that there has been nothing 
like it since, and that all subsequent formations have shown 
wider and wider divergences from the carboniferous type.* 
30. Doubtless there have been loose statements erroneously 
made concerning the complete universality of ancient deposits. 
But, allowing for this, it cannot be denied that the crystalline 
schists, slate rocks. Devonians, and carboniferous strata, were 
spread on both sides of the equator, and around the globe more 
uniformly than can be paralleled since.f 
31 Formations apparently similar may not have been strictly 
contemporaneous, and dissimilar formations may have been so. 
Along the same line of river or coast there is being deposited 
at the same time gravel in one place, sand in another, mud in a 
third, all dependent on the amount of force in the stream and 
the nature of the banks or coast. Identity in composition is not 
No coal-fields, to last even a single century, are now growing at the 
mouths of our rivers ; no metallic veins are spreading through the rocks that we 
can explore ; no great catastrophe breaks down the barriers of seas, or opens 
picturesque glens through the ridges of the mountains.”— PhiHipps, Origin of 
Life , p. 166 . 
t As a rule, the older the reck is in the history of the world, the greater 
will be the area over which its chemical composition and character remain 
unaltered.” — Haughton, Manual , p. 88. 
