DECEMBER 24, 1897. ] 
published until 1815. The appearance of 
this map marked ‘a distinct epoch in strati- 
graphical geology, for from that time some 
of what are now the most familiar terms in 
geological nomenclature passed into com- 
mon use.’ Smith also published geological 
maps, on a larger scale, of the English coun- 
ties and a series of horizontal sections across 
different parts of England. 
The sizth and last lecture deals with 
the further development of stratigraphical 
geology along lines laid down by Smith and 
his distinguished contemporaries, who had 
applied the criteria derived from fossils 
with such success. Smith’s researches, as 
we have seen, did not include rocks older 
than the Coal Measures. The great mass 
of earlier strata known in the old classifi- 
eation as Greywacke, or Transition, rocks 
were regarded in their generally disturbed 
and poorly fossiliferous condition to be be- 
yond interpretation by the principles of 
Cuvier, Brongniart and Smith, until Mur- 
chison and Sedgwick, for the most part, 
‘working independently of each other in 
Wales and in the border counties of Eng- 
land,’ succeeded in establishing a definite 
order among the oldest fossiliferous for- 
mations, thus adding the Devonian, Si- 
lurian and Cambrian chapters to the 
geological record. 
Murchison (1792-1871), after several 
years of investigation of the Secondary 
rocks of England and the Continent, and 
some preliminary work with Sedgwick upon 
the old rocks of the northern counties of 
England, began in 1831 in Wales and the ad- 
joining counties of England his epoch-mak- 
ing study of the strata below the Old Red 
Sandstone. Starting with thatalready known 
and easily recognizable horizon he estab- 
lished a series of underlying divisions which 
he found to be characterized by peculiar 
fossils. To this assemblage of formations, 
which he divided into an Upper and a 
Lower series, he gave the name of Silurian 
System. He recognized its conformity to 
SCIENCE. 931 
the Old Red Sandstone, but wrongly thought 
it to rest unconformably upon the older 
series of greywacke. Murchison also 
worked out the lithological character of 
these old rocks, observing eruptive ma- 
terials among them, some of which he 
clearly saw were intrusives, while others 
he recognized to be lavas and ashes. His 
first communication upon this subject was 
made to the Geological Society of London in 
1831, his great book, ‘ The Silurian System,’ 
appearing in 1838. Theauthor tells us that 
even before the advent of this volume his re- 
markable results had become widely known 
and ‘ within a few years the Silurian System 
was found to be developed in all parts of 
the world,’ Murchison’s work furnishing 
the key to its interpretation. 
Sedgwick (1785-1873), almost from the 
very beginning of his career, devoted his 
energies to the ancient rocks, his earlier 
publications, however, showing strong 
leanings to the Wernerian school. He 
soon parted with these views and early 
came to a true perception of geological 
principles which he applied in a study of 
the older formations of northern England. 
The author tells us that though fossils had 
been found in the rocks Sedgwick did not at 
first make use of them for purposes of strati- 
graphical classification, but ascertained 
the succession of the great groups of strata 
upon. lithological grounds alone. He, as 
well as Murchison, recognized volcanic 
rocks to form part of the greywacke rocks 
of North Wales and soon ‘ succeeded in dis- 
entangling their structure and ascertained 
the general sequence of their principal sub- 
divisions.’ At this period, however, his in- 
vestigations were of far less significance in 
the field of general stratigraphy than Mur- 
chison’s, since he had not determined the 
relation of his rocks to any well recognized 
horizon and had made no use of fossils for 
correlative purposes. Later investigations 
showed that the upper part of what Sedg- 
wick termed the Cambrian system con- 
