278 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
this material being derived from a spiral nebular mass formed 
through the close approach of two large bodies. As the nuclear earth 
grew in dimensions, so also was increased the gravitative pressure, 
gradually developing central heat which spread to the surface and 
there broke out in a long period of volcanic activity. 
Our knowledge of glacial climates had its origin in the Alps, the 
land of magnificent scenery and marvelous glaciers, through the 
work of Andreas Scheuzer, early in the eighteenth century. This 
was at first only a study of the interesting local glaciers, but out 
of it gradually came about, especially through the studies of De 
Saussure, Hugi, Venetz, Charpentier, Schimper, and Louis Agassiz, 
the application of conditions observed in the Alps to the very widely 
distributed foreign bowlders known as erratics and the hetero- 
geneous accumulations of sands, clays, and bowlders called tills. 
The engineer Venetz in 1821 pointed out that the Alpine glaciers 
had once been of far greater size, and that glaciation had been on 
a scale of enormous magnitude in some former period. By degrees 
the older conception that the erratics and tills were of flood, river, or 
iceberg origin gave way to the theory of colder climates and glaciers 
of continental extent. It was shown that the reduced temperature 
was finally succeeded by greater warmth, and that in the wake 
of the melting glaciers the land was strewn with erratics, with thick 
accumulations of heterogeneous rocks deposited at the edge of ice 
sheets and known as moraines, and with great fans of bowlder clays 
and sands, all of this being the diluvium or deluge material of the 
older philosophers and the drift or tills of modern students of earth 
science. 
Throughout more than a century of study we have learned how 
glaciers do their work and what results are accomplished by their 
motion plus the action of temperature, air, and water. ‘The present 
geographic distribution of the glaciers, together with that of the 
glacial deposits, shows us that during the Pleistocene or glacial 
period the temperature of the entire earth was lowered. We also 
know that this cold period was not a uniformly continuous one, but 
that during the Pleistocene there were no less than four intermediate 
warmer climates, so warm indeed that during one of them lions and 
hippopotamuses lived in western Europe along with primitive man. 
We may now be living in another interglacial warm period, though 
more probably we are just emerging from the Pleistocene ice age. 
Figure 1 gives the known distribution of Pleistocene glacial materials. 
With the reduction of temperature, great variations also took place 
in the local supply of moisture, in the number of dark days, and in 
the air currents. How great these changes were in Pleistocene 
time is now being revealed to us through the work of the geologists, 
paleontologists, and ethnologists of Europe, where this record is far 
