AND DISTRIBUTION OF TRILOBITES. 285 
Devonian system of stratification differ entirely from the Trilo- 
bites of the other English strata, excepting only Cal. Latreillii 
and granulata (Phacops macrophthalmus). It contains Cal. acci- 
pitrina, levis, granulata, Latreillii (species of Phacops), Stern- 
bergii, Harpes macrocephalus, Asaphus granuliferus, Brontes fia- 
bellifer, and other species of this genus. Of all these, the two 
species, Ph. macrophthalmus and Brontes flabellifer, already oc- 
cur, although rarely, in the Wenlock limestone; one species, 
As. granuliferus, on the other hand, is a Trilobite of the moun- 
tain limestone, whilst the very remarkable Cal. Sternbergii and 
Harpes are, together with the others, genuine Devonian Trilo- 
bites. The distribution of the Trilobites confirms, therefore, as 
well as that of other fossil animals, the separation of the trans- 
ition rock into four principal subdivisions; into a lower and 
upper Silurian, into a Devonian system, and into mountain lime- 
stone. 'They may all be characterized by species, and even by 
genera of Trilobites peculiar to them. But however distinctly 
these subdivisions differ from one another by the nature of their 
constituent rocks and fossils, this is not the case even on British 
soil—everywhere. The fact of a rock of an entirely different 
character being deposited from the ocean in the south-western 
part of the present country, pretty nearly at the same period 
with the red conglomerate and sandstone formation in the west 
and north-west of England, is in favour of the view that local 
influences were not excluded during the formation of the palzo- 
zoic rocks. Conglomerate formations naturally possess the most 
local character of all rocks deposited from water ; clay formations, 
and next to them fine granular sandstone formations the most 
general. Limestones hold an intermediate rank; they are fre- 
quently wanting under circumstances that admit of no possi- 
bility of a previous destruction of them. I allude to Sweden 
and Norway. The English transition formation exhibits a simi- 
lar relation to that of the neighbouring island of Ireland. The 
transition rock in the northern part of this island, in the county 
of Tyrone (compare Portlock’s ‘ Geology of Londonderry,’ 1843), 
projects in two small districts from amidst the old red sand- 
stone; these two districts are in the parishes of Pomeroy and 
Desserlcreal in the vicinity of Dungannon. Both are formed of 
grauwacke slate which resembles very much the Caradoc sand- 
stone; limestone deposits are wanting; the stratification is dis- 
VOL. IV. PART XIV. a | 
