234 
PASSING OF MURCHISON’S SILURIA 
mystical Transition rocks geological classification and stratigraphic 
progress might have remained no farther advanced today than it 
was a hundred years ago. Siluria was launched without waiting 
for final word, fully discussed in all its phases for that early 
day, and finally, after long and useful career, gave way in the 
natural course of events, to more refined concepts of terranal 
grouping and delimitation. Its proposal and wide use in the first 
half of the last century were most appropriate, but it long since 
served its every purpose. 
The basic purpose of Siluria was to give something of a time 
measure to older geologic sedimentation. When a more accurate 
standard became available the older measure had to give way to 
the newer. The ideal in this respect is, of course, some such unit 
as the year, or the century, in human affairs. This can hardly 
be in geologic computation; but the nearest approach to some 
distinctive unit is a desideratum. Now Siluria is not a periodic 
division by any means, but a division of higher taxonomic rank, 
one including a number of periods. As such it should have a time- 
span as nearly proportional to those of the other divisions of the 
same rank. This it does not seem to have either in its original 
signification or in its later restricted sense. 
Bearing directly upon this phase of the Silurian question are 
some recent estimations on the length of geological time, which 
the several periodic divisions represent according to an adjustment 
of the processes of organic evolution. The majority of the recog¬ 
nized periods seem to respond to an evaluation of about 25 mil¬ 
lions of years. The Siluric, or Upper Silurian, and the Devonic 
periods appear to have only about one-half of these values. Since 
the faunas of these divisions are closely related, and are with 
difficulty separated, and since together they hold to all intents and 
purposes a time equivalent equal to every other period it becomes 
almost obligatory to readjust the terranes to meet these conditions. 
Such a section could be known as the Yorkic, or Yorkian, Period, 
in commemoration of what is perhaps its most complete terranal 
representation in the world — the New York section of these 
rocks. Those who prefer may still use the terms Devonian and 
Silurian as alternate titles for the upper and lower parts of the 
rock section of this period. But Yorkic more nearly gives the 
