On the three principal tests of relative age--superposition, mineral character, and fossils--Change of mineral character and fossils in the same continuous formation--Proofs that distinct species of animals and plants have lived at successive periods--Distinct provinces of indigenous species--Great extent of single provinces--Similar laws prevailed at successive geological periods--Relative importance of mineral and palaeontological characters--Test of age by included fragments--Frequent absence of strata of intervening periods--Principal groups of strata in western Europe 96 CHAPTER X. CLASSIFICATION OF TERTIARY FORMATIONS.--POST-PLIOCENE GROUP. General principles of classification of tertiary strata--Detached formations scattered over Europe--Strata of Paris and London--More modern groups--Peculiar difficulties in determining the chronology of tertiary formations--Increasing proportion of living species of shells in strata of newer origin--Terms Eocene, Miocene, and Pliocene--Post-Pliocene strata--Recent or human period--Older Post-Pliocene formations of Naples, Uddevalla, and Norway--Ancient upraised delta of the Mississippi--Loess of the Rhine Page 104 CHAPTER XI. NEWER PLIOCENE PERIOD.--BOULDER FORMATION. Drift of Scandinavia, northern Germany, and Russia--Its northern origin--Not all of the same age--Fundamental rocks polished, grooved, and scratched--Action of glaciers and icebergs--Fossil shells of glacial period--Drift of eastern Norfolk--Associated freshwater deposit--Bent and folded strata lying on undisturbed beds--Shells on Moel Tryfane--Ancient glaciers of North Wales--Irish drift 121 CHAPTER XII. BOULDER FORMATION--_continued_. Difficulty of interpreting the phenomena of drift before the glacial hypothesis was adopted--Effects of intense cold in augmenting the quantity of alluvium--Analogy of erratics and scored rocks in North America and Europe--Bayfield on shells in drift of Canada--Great subsidence and re-elevation of land from the sea, required to account for glacial appearances--Why organic remains so rare in northern drift--Mastodon giganteus in United States--Many shells and some quadrupeds survived the glacial cold--Alps an independent centre of dispersion of erratics--Alpine blocks on the Jura--Recent transportation of erratics from the Andes to Chiloe--Meteorite in Asiatic drift 131 CHAPTER XIII. NEWER PLIOCENE STRATA AND CAVERN DEPOSITS.