932 
tained the same organic remains as the 
Lower Silurian formations defined by Mur- 
chison, and thus arose a permanent mis- 
understanding between these two old friends 
and leaders in English geological thought. 
While this dispute was in progress Bar- 
rande made his remarkable investigations 
in the Bohemian basin, where he not only 
recognized the equivalents of Murchison’s 
Upper and Lower Silurian, but also found 
a still older group of strata containing 
forms similar to those occurring in Sedg- 
wick’s Cambrian system. To the latter 
fauna he gave the name of First or Pri- 
mordial fauna. The consensus of geolog- 
ical opinion, on the grounds of priority, 
to-day regards as Lower Silurian the Upper 
Cambrian of Sedgwick. Sedgwick’s classi- 
fication, however, in these old and dis- 
turbed rocks has proved of vast importance 
in the elucidation of the ancient sediments, 
and the succession of strata observed by 
him has continued with hardly a modifi- 
cation to the present day. 
A no less important work pursued by 
Sedgwick and Murchison together in the 
days before their estrangement was the 
determination of the Devonian. Its refer- 
ence in the absence of stratigraphical data 
to a position between the Silurian and 
Cambrian was based mainly upon the pale- 
ontological work of Lonsdale, who pointed 
out that the fossils in its lower portions 
showed an affinity to the Silurian while 
those in its upper parts were closely allied 
to the Carboniferous faunas. 
The pre-Cambrian rocks now remained 
to be studied and deciphered. The paleon- 
tological criteria were no longer available 
and many difficulties presented themselves. 
“The first memorable onward step,’’ accord- 
ing to Geikie, ‘“‘ was taken in North America 
by Logan (1798-1875).”” Some study had 
already been given to these old rocks, but 
Logan was the first to attempt to establish 
a chronological sequence among them. To 
him we owe the names Laurentian and 
SCIENCE. 
(N.S. Von. VI. No. 156 
Huronian, and although his results have 
been much modified by subsequent ob- 
servers his work marks a distinct advance 
in this field of stratigraphical geology. 
The first recognition of the wide signifi- 
cance of glaciers as geological factors of 
more than local importance must be ac- 
corded to Agassiz (1807-1873). He was 
the first to offer a satisfactory explanation 
for the so-called erraties, which were found 
distributed over the Swiss plain and the 
flanks of the Jura mountains. Contrary 
to the preconceived notions of the day, he 
held that the Alpine ice once extended 
over the area and that it was an expla- 
nation of a former period of extreme cold. 
His further researches in England, where 
he found similar phenomena, convinced 
him that the great extension of ice was 
connected with the last great geological 
change on the surface of the globe. These 
teachings of Agassiz, which to-day, in all 
their essential elements, have been gener- 
ally accepted, place his name, according 
to the author, as that of ‘ the true founder 
of glacial geology.’ 
The attempts at geological classification 
upon lithological grounds, which had been 
pushed to such an extreme by Werner and 
his followers, greatly declined after the 
marvelous impetus which the study of or- 
ganic remains brought to the science of 
geology. But the investigation of rocks in 
their mineralogical aspects was not to be 
permanently abandoned. ‘The invention of 
the famous prisms of Iceland spar by Nicol 
and the cutting of thin sections introduced 
a new element into geological investigation, 
but it was not until Sorley extended this 
method by the more systematic examination 
of thin sections that microscopical petrog- 
raphy became recognized in the field of 
geological research. The publication, in 
1858, of his memoir ‘On the Microscopic 
Structure of Crystals’ marks one of the most 
prominent epochs in modern geology. There 
was at first much opposition to this method 
