communicated personally or by letter with not a few of the
geologists, English and foreign, who have taken part in these
researches. Besides explaining in the present volume the results of
this inquiry, I shall give a description of the glacial formations
of Europe and North America, that I may allude to the theories
entertained respecting their origin, and consider their probable
relations in a chronological point of view to the human epoch, and
why throughout a great part of the northern hemisphere they so
often interpose an abrupt barrier to all attempts to trace farther
back into the past the signs of the existence of Man upon the
earth.

In the concluding chapters I shall offer a few remarks on the
recent modifications of the Lamarckian theory of progressive
development and transmutation, which are suggested by Mr. Darwin's
work on the "Origin of Species by Variation and Natural Selection,"
and the bearing of this hypothesis on the different races of
mankind and their connection with other parts of the animal
kingdom.

NOMENCLATURE.

Some preliminary explanation of the nomenclature adopted in the
following pages will be indispensable, that the meaning attached to
the terms Recent, Pleistocene, and Post-Tertiary may be correctly
understood. [Note 1.]

Previously to the year 1833, when I published the third volume of
the "Principles of Geology," the strata called Tertiary had been
divided by geologists into Lower, Middle, and Upper; the Lower