498 
GEOLOGY: J. BARRELL 
lacustrine beds and other beds laid down in shifting, shallow, and vari- 
able bodies of water, are dominantly flu via tile in origin; laid down over 
river flood plains by streams in times of flood, exposed to air in times of 
drought. They record in this way the existence of an alternation of 
seasons of rainfall and drought — a climate with an arid season, but not 
an arid climate. This type of climate is best defined as semi-arid, and 
is existent over broad areas at present, especially in much of the torrid 
zone. Such a climate is to be sharply distinguished in thought, on the 
one hand, from that of a typically humid character, such as now pre- 
vails in northwestern Europe and northeastern North America; and, 
on the other hand, from truly desert climates, such as prevail in cen- 
tral Asia or northern Africa. The conclusion as to dominantly fulvia- 
tile origin, — similar to the mode of origin of the sediments now accu- 
mulating within the Great Valley of California, or on the Plains of 
Mesopotamia, is also to be sharply distinguished from the conception 
of great and permanent lakes, as well as from the opposite conception of 
torrential and eolian conditions in desert basins. Thus the present in- 
terpretation is distinctly different, both in regard to climatic and physio- 
graphic conditions of origin, from either the prevailing British or the 
German conceptions, but approaches nearest to that given by Goodchild. 
The character of this interpretation may be called American, for in 
its main lines the writer does not stand alone. W. M. Davis, Hatcher, 
and others have shown the general importance of flood plain deposition. 
Especially may be mentioned the work of Grabau who has independently 
reached similar conclusions in regard to the importance in certain 
Paleozoic formations of flu via tile deposition. He has studied the Old 
Red Sandstone in the field and, though he has not published on that 
subject, has publicly stated that his conclusions are the same as those 
here expressed. More recently Schuchert has reviewed the evidence 
and expresses similar conclusions, though the part played by rivers as 
distinct from lakes is not so sharply differentiated as in the papers of 
Garbau and the writer.^ 
In closing we may draw a picture of the geography of Great Britain 
in Lower Devonian times. The region was then a part of a continent 
which extended an unknown distance northward and westward. Be- 
yond the northwestern side of Great Britain extended a mountain sys- 
tem. The region of Great Britain was, in this newer interpretation, 
made up of a sequence of river basins separated by minor ranges of 
mountains, the whole marginal to the greater mountain system. Sedi- 
ment was brought into these basins by rivers from the bordering up- 
lands and from the more distant regions to the northwest. The excess 
