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