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Geology of High Teesdale. 187 
Secrion LY. 
On the Origin of the Trap Rocks of High Teesdale. 
Havine, in the two preceding Sections, considered the prin- 
cipal facts connected with the structure of High Teesdale, and 
the phenomena arising out of the association of trap rocks with 
the regular strata of the district; it only remains for me to con- 
sider the probable origin of this anomalous class of rocks. 
1. In the first place, they are, in mineralogical composition, 
all nearly identical with undoubted volcanic products. This asser- 
tion is sufficiently proved by the details given in the early part 
of the preceding Section. On the contrary, the regular strata of 
the district are unquestionably of aqueous origin, many of them 
consisting, almost exclusively, of the petrified remains of zoophytes, 
shells, and corals, and they differ altogether in composition and 
structure, from every variety of trap. This entire contrast makes 
it probable that the different formations have had a different 
origin. 
2. The lower dyke which crosses Eglestone Burn, is con- 
nected, at one extremity, with a mass of trap on the north bank 
of the Tees, and in the other direction, it is prolonged into the 
eastern moors. In its structure, and in its position among the 
regular beds, it differs in no essential respect from the dykes 
described in a preceding Paper (supra, p. 21.) and it is probably 
connected with some of them. If then we have good evidence 
for the igneous origin of the dykes in the coal-field, it follows, 
that the lower dyke in Eglestone Burn, and the mass of trap 
on the north bank of the Tees, must be of igneous origin. 
3. The connexion of this lower dyke, with the mass of trap on 
the north bank of the Tees, seems to prove, that it must have 
been formed by injection from below. 
AA2 
Origin of the 
Trap, igneous. 
