Page:A Treatise on Geology, volume 1.djvu/9



N revising this Edition, I have employed, as the basis of classification for successive geological periods, those great natural associations of the forms of life which are expressed by the terms Palæozoic, Mesozoic, and Cainozoic. In subdividing the great classes of strata which were formed in these successive "life-periods," no material change has been found necessary, except in regard to the Palæozoic formations. Among these a much improved distribution has been effected, by labours commenced in Britain, in 1831, by Murchison and Sedgwick, and since extended by these and other geologists over great part of the globe. If some of the questions which these researches have raised are as yet only provisionally answered, they are placed in a form which may probably lead to sound and comprehensive solutions. (Vol. I., Palæozoic Strata.)

Among the many objects of geological inquiry which have been successfully prosecuted of late years, the following must be distinguished:─

The beautiful structure of slate has acquired