234 The America/)) GeoUxjint. April, 18% 
paralU'l of the EuropeMii (Jambriaii, it is highly probable that 
some of the eruptive rocks whicli have been found closely as- 
sociated with the Cambrian in Wales and P^ngland are of so 
late a date that they fall within Cambrian time. These seem 
to be represented in the Taconic in America. The exact re- 
lation that they bear to the Olenellus horizon has been an 
American question with different views, as in Europe, but in 
America the earliest principal disturbance apparentl}' followed 
this fauna, as will appear later. 
The position of the Olenellus zone has been thus summarily 
defined by Van Hise : 
Placed ill tin- Aliionkijiii. uiulfr this (icfliiit ion | i. c assumiiiji- tlu' 
Olenellus /one lis tlie base i>r the Cambria.n. N. H. W.] are 11.000 feet 
of quai tzytes conformably below the Olenellus in the Wasatch: 10,000 
feet of arjiillytes, sandstones, quartzytcs. and coiiiiiomerates conforma- 
bly beneath the Olenellus in liritish Columbia: 12.000 feet of sandstones, 
shales and limestones uncomformably beneath the htwest known Cam- 
brian in the Grand canyon t>f the Coloradt): ;i similar series of rocks 
unconformably beneath the Cambrian in T^lano county, Texas, a series 
unconformably beneath the upper Cambrian in the Adirondacks, and 
the rocks of St. Mary's and PlaceiUia bays, Newfoundland, which are 
unconformably below Tjower ("ambrian strata. Van Ilim. Cofviioiion 
Papers, p. 4'>^>- 
It is only intended by this brief review to call attention to 
the stratigraphic position of the Olenellus fauna in those por- 
tions of the globe where it has been most carefull}'^ deter- 
mined. It certainly is a convenience from a paleontological 
standpoint to recognize definite f'aunal planes. It may be, 
therefore, an aid to the progress of geological research to 
refer to the plane of the Olenellus fauna as a marked and well 
established datum; and in the pi'esent state of stratigraphic 
paleontology such an assumption may serve a good purpose 
for a trial hypothesis: but it should be remembered that there 
are many lines yet to be followed out and many regions yet 
unexamined. The Paradoxides and the Olenellus faunas may 
be fount! to be closely related, and perhaps to blend, as sug- 
gested by Matthew. It should also be remembered that if it 
become agreed to limit the Cambrian at the Olenellus zone 
the underlying conformable clastic strata, down to the great 
non-conformity, are not excluded from the 'J'aconic. 
