An examination of the old red sandstone, as is seen in Herefordshire, will aid us in forming a notion of the time required for its production. It is composed of fragments obtained by the disintegration of more ancient rocks, which, by a long process of rolling together in a breaking sea, or in the bed of a rapid current, have lost all their angles. The pebbles, thus worn, have at length settled,--the heaviest lowest,--and the whole has been consolidated into firm rock. "In many places," says Dr. Pye Smith, "the upper part of this vast formation is of a closer grain, showing that it was produced by the last and finest deposits of clayey and sandy mud, tinged, as the whole is, with oxides and carbonates of iron, usually red, but often of other hues. But, frequently, the lower portions, sometimes dispersed heaps, and, sometimes, the entire formation, consist of vast masses of conglomerate, the pebbles being composed of quartz, granite, or some other of the earliest kinds; and thus showing the previous rocks, from whose destruction they have been composed. Let any person first acquire a conception of the extent of this formation, and of its depth, often many hundreds, and, sometimes, two or three thousand feet; (but such a conception can scarcely be formed without actual inspection;) then let him attempt to follow out the processes which the clearest evidence of our senses shows to have taken place; and let him be reluctant and sceptical to the utmost that he can, he cannot avoid the impression that ages innumerable must have rolled over the world, in the making of this single formation."[27] Here, Fishes are added to the Invertebrate Animals. A sort of Shark with the mouth terminal, instead of beneath the head, was the earliest representative of this class. But closely following on this, were some curious species, enveloped in plate mail, and remarkable for the singularity of their forms, as the _Cephalaspis_ and the _Pterichthys_. [Illustration: CEPHALASPIS.] This great period passed away, and was succeeded by that of the Carboniferous deposits, indicative of a vast change in the physical character of the earth's surface and atmosphere. This change of character may be briefly summed up as consisting of an immense abundance of lime in the ocean, and of an equally vast charge of carbonic acid in the atmosphere.