278 ILLINOIS ACADEMY OF SCIENCE 
sition, and to the abundance of slowly decaying vegetable mat- 
ter at the place of their formation. The Lower Cretaceous 
rocks of the Upper Yukon are almost exactly like those just 
cescribed, except that they are much thicker. In 15,000 feet 
of beds exposed in a single section no bed of any color except 
black and dark gray could be seen. 
Passing over the Triassic oily shale and limestone, and the 
Permian white limestone-marine strata the climatic significance 
ot which may be different, although as yet uncertain—we 
come to the National River formation of Pennsylvanian age. 
This consists of several thousand feet of blackish shale and 
silt rock alternating with dark muddy graywackes and gray- 
wacke conglomerate. The sandy layers contain abundant fresh 
particles of feldspar, slate and mica, and shreds of woody fiber 
—the appropriate product of a moist and cool or temperate 
climate, 
The Mississippian formations, being marine, are somewhat 
less positively interpreted, but even in them the prevailing rock 
is black shale rich in carbonaceous matter, and it is significant 
that the many intercalated beds of black limestone are devoid 
of reef corals or other organisms that are confined to tropical 
waters. 
The Devonian consists largely of black coaly shales, cherts, 
dark limestones and unweathered volcanic greenstones, all of 
which are consistent with the supposition of a moist temperate 
or cold climate. In certain beds, perhaps of Devonian age, the 
shales and cherts are associated with a bed of glacial tillite 
containing striated subangular boulders. 
The Silurian consists in part of massive gray or white lime- 
stones and dolomites, and so probably constitutes an exception 
to the monotony of the black formations. The return seems to 
be made to the Alaskan type, however, in the Ordovician where 
basaltic breccias and tufas have remained to this day with 
scarcely a trace of chemical decay. Even the olivine crystals 
preserve clear, sharp outlines, and the feldspars show no cloud- 
ing. 
In rocks which are probably not younger than early Paleo- 
zoic, and may prove to be of Lower Cambrian age, other 
Leds of glacial tillite have been found in two places in associa- 
