Another source of the widely-spread error which prevailed concerning the origin of basalt, was the failure to recognise the nature of the alterations which take place in the character of rock-masses in consequence of the passage through them, during enormous periods of time, of water containing carbonic acid and other active chemical agents. The casual observer does not recognise the resemblance which exists between certain ornamental marbles and the loose accumulations of shells and corals which form many sea-beaches; but close examination shows that the former consist of the same materials as the latter, bound together by a crystalline infilling of carbonate of lime, which has been deposited in all the cavities and interstices of the mass. In the same way, as we have already seen, the vesicles and interstices of heaps of scoriæ may, by the percolation of water through the mass, become so filled with various crystalline substances, that its original characters are entirely masked. But the progress of chemical and microscopic research has effectually removed these sources of error. Many rocks of aqueous origin, formerly confounded with the basalts, have now been relegated to their proper places among the different classes of rocks; while, on the other hand, it has been shown that the chemical and physical differences between the ancient basalts and the modern basic lavas are slight and accidental, and their resemblances are of the closest and most fundamental character. [Sidenote: VOLCANIC ORIGIN OF 'TRAP ROCKS.'] The notion of the aqueous origin of basalt, which was so long maintained by the school of Werner, has now been entirely abandoned, and the so-called 'trap-rocks' are at the present day recognised as being as truly volcanic in their origin as the lavas of Etna and Vesuvius. There is, however, a vestige of this doctrine of Werner, which still maintains its ground with obstinate persistence. Many geologists in Germany who admit that volcanic phenomena, similar to those which are going on at the present day, must have occurred during the Tertiary and the later Secondary periods, nevertheless insist that among the earlier records of the world's history we find no evidence whatever of such volcanic action having taken place. By the geologists who hold these views it is asserted that while the granites and other plutonic rocks were formed during the earlier periods of the world's history, true volcanic products are only known in connection with the sediment of the later geological periods. Some geologists have gone farther even than this, and asserted that each of the great geological periods is characterised by the nature of the igneous ejections which have taken place in it. They declare that granite was formed only during the earliest geological periods, and that at later dates the gabbros, diabases, porphyries, dolerites and basalts, successively made their appearance, and finally that the modern lavas were poured out.