on the Physical Structure of Devonshire. 317 



The subjacent black limestone has indeed no exact parallel in En- 

 gland, its organic remains being for the most part peculiar and un- 

 described ; they are all apparently of marine origin. Among them are 

 two genera of large, transversely ovate bivalve shells, one of which 

 has a strong resemblance to a Possidonia of the Yoredale series 

 of the Mountain Limestone (Phillips). Another is like, but not 

 identical with Gervillia laminosa (Phillips). Chambered shells also 

 occur, some of which are considered to be Goniatites, a genus which 

 has never yet been found in the Silurian or older rocks, but is most 

 characteristic of the carboniferous system. 



In mineral characters this black limestone approaches closely to the 

 calp of Ireland, which though now clearly acknowledged to be a part 

 of the carboniferous group, is nearly devoid of characteristic fossils. 

 As the whole formation is of enormous thickness and exhibits no 

 plants with distinct specific characters in its lower parts, and as the 

 black limestone contains no species of shell absolutely identical with 

 fossils of the mountain limestone, the authors consider the base line of 

 the series as in a position not yet completely ascertained j though they 

 distinctly prove that it never passes down into the older rocks on which 

 it rests. As however, the upper group contains a fine series of 

 vegetable fossils, everyone of which agrees specifically with true car- 

 boniferous plants, they have no hesitation in placing these culm mea- 

 sures on the same parallel with the true carboniferous series of Great 

 Britain. The evidence of fossils is in favour of the conclusion, and 

 the sections, instead of opposing, confirm it j in short the culm-bearing 

 beds of Devon are identical with the coal measures of Pembrokeshire 

 both in mineral character and organic remains. 

 Chap. V. — Granite of Dartmoor, &c. 



The jointed structure of this rock is described in detail, and the joints 

 are shown to agree in their direction with those described by Messrs. 

 Fox, Enys, and other geologists in Cornwall. The authors also con- 

 firm a remark of Dr. Boase that the same master joints often affect the 

 granite and bedded rocks near it: they show that the granite has in 

 some places broken through the stratified formations without much 

 changing their strike ; hence the successive members of the culm 

 measures abut against the granite on the north-west side of Dart- 

 moor. In all such cases, following the beds along the line of strike, 

 they are changed in structure as they approach the granite, the sili- 

 ceous bands being converted into quartz rock, the shales into 

 Lydian stone, felspar, porphyry, &c. They regard thesefacts as perfect 

 proofs of the metamorphic nature of the rocks in contact with the 

 granite of Devon. Lastly, they describe granite veins and elvan 

 dykes as traversing the Culm Measures. 



The conclusion is, that no rocks in Devon or Cornwall are older 

 than the Upper or Middle Cambrian j that a magnificent develop- 

 ment of the Upper Cambrian terminates in the ascending order about 

 the base of the Silurian system ; that these rocks are surmounted by 

 an immense culmiferous trough, the upper portion of which is identical 

 in fossils with the upper division of our coal measures; and that the 

 granite is posterior to all these, but probably anterior to the new red 

 sandstone. 



