Base of the Taconic or Loirer Camhridv. — Wlnchell. 281 
Throughout Britain, therefore, in late years, the paleontolog- 
ic base of the Canihrian has been taken to be the Olenellus 
horizon, although evident clastic strata, long called Cambrian, 
occur widel}' below the Olenellus horizon. 
It appears, from an examination of the literature bearing 
upon the Cambrian of Britain, that the time of the C'ambrian 
was subject to volcanic outbreaks, as represented by A. Geikie 
and by Dr. Hicks. In some places this was the cause of sig- 
nificant variation in the nature of the sediments, as well as 
of changes of level of the land, causing the submergence of 
areas that had before been dry, and bringing the Paradox- 
ides beds and later the Olenus beds non-cfrnformable upon the 
Archean. and even upon older parts of the Cambrian. The 
principal disturbance of this nature seems to have taken place 
early in Cambrian time. Just at what epoch the Olenellus 
fauna was introduced, at the different places at which it has 
been discovered, there is as yet not sutlicient evidence to af- 
firm. It may be that the life of the Olenellus fauna was very 
long, and that when it was exterminated at one point, by vol- 
canic cf>nditions in the ocean, it continued to flourish at oth- 
ers, or tiiat it returned again to its former habitat on the 
return of favorable conditions. Another principal event in 
the time of the Cambrian, in Britain, seems to have been that 
which separated the Olenus strata so markedly from the older 
portions of the Cambrian. This in some places brings the 
Olenus beds directly upon the Olenellus or pre-Olenellus 
strata. 
In Scandinavia and the Baltic region the (ambrian forma- 
tion is greatly redu<-ecl in thickness, and the Olenellus fauna 
is found near the base, which is marked by a striking non- 
conf(u'mity upon the gneissose Archean. There has not been, 
so far as observed, any proposition to separate the lowest of 
these strata from the Cambrian. The occurrence of Olenellus 
near the basal beds in Scandivavia, where the most evident 
succession of parts appears, and where the lower faunal changes 
were made out first with exactness, seems to have been the 
initial fact which inspired the idea of limiting the Cambrian 
with the Olenellus beds. 
In the Salt range of India, according to recent w(U'k by 
