Geology of the South-east of Devonshire. 465 



impregnated with calcareous matter ; and beds of contemporaneous trappean ash run parallel with the 

 limestones from Launceston to Oakhampton. In South Devon the coral limestones are in many places 

 superincumbent on great sheets of volcanic materials, with which in some instances, as at North Whil- 

 borough, they alternate ; and the compact greenstones of South Devon contain a variable, but in all 

 cases, a considerable proportion of lime. This association may be accounted for by supposing, either 

 that subsequently to such eruptions the waters may have held a greater quantity of lime in solution ; 

 or that the bed of the sea was altered and placed under conditions more favourable for the production 

 of limestone masses. 



Very little personal examination of (he principal limestone mass of South Devon will satisfy any one, 

 that it has resulted from the labours of Polypi ; in other words, existed as a coral reef at the close of 

 the transition period. In the parish of East Ogwell each bed is entirely composed of branching corals in 

 the positions in which they grew ; and in the parish of Denbury the partings between the beds of lime- 

 stone are of volcanic sand, which is easily removed, and we then obtain an upper surface studded with 

 projecting stars and branches, as perfect as when the sand was thrown upon them and suspended the 

 labours of the Polyps. Among existing zoophytes the lamellated and cellulated orders seem to be the 

 principal agents in constructing reefs, and their analogues mainly compose the Devonshire limestone. 



The calcareous rocks of South Devon, in their structure, as well as in the position, preservation, and 

 grouping of their corals, afford abundant evidence that their production was analogous to that of modern 

 coral reefs ; and in their general position they agree with the reefs of the southern hemisphere, where 

 the Polyps raise their habitations on the summits or slopes of submarine hills to a level with the surface 

 of the water, and hence the table form of land which the limestone masses still preserve. 



A local association of certain generic, and even specific forms is often to be observed ; thus the lime- 

 stone of Newton abounds in CyathophjllcB and CoscinoporcB ; that near Chercombe Bridge seems to have 

 been almost exclusively constructed by Favosites polymorpha and F. spongites : whilst about Denbury, 

 Favosites alveolaris and other allied species are most abundant. 



The organic remains contained in the rocks of South Devon are very numerous, and in beautiful pre- 

 servation. A critical examination of a collection of these fossils by Mr. Lonsdale led to very interesting 

 results connected with the older rocks of this country, and the rectification of an error as to their age 

 and place. The generally adopted opinion at that time was, that the slates and limestones of South 

 Devon were of the same age with the slates of the Berwyns and the limestone of Bala. Mr. Lonsdale's 

 correction was arrived at by the only safe guide, — zoological evidence, by which he established, from the 

 intermediate character of the forms, between those of the true mountain limestone on one hand, and those 

 described by Mr. Murchison in the Silurian System on the other, that the older South Devon limestones 

 were the equivalents of those deposits which in other parts of England underlie the mountain limestone. 

 The old red sandstone is in general mineralogically different, and sparingly fossiliferous ; and until 

 the publication of Mr. Murchison's work, we were but little acquainted with any forms from these thick 

 deposits. This work contains descriptions and figures of seventeen fishes, twenty-seven shells, and one 

 crustacean, in all forty-five species. It is proper only to state, that of these not one has as yet been 

 found in South Devon, whilst, on the other hand, they strikingly resemble such as are contained in the 

 arenaceous deposits in North Devon, which underlie the carbonaceous beds, and so far agree in position 

 with the old red sandstone of the opposite coast of Pembrokeshire. 



In no other part of England do we appear to have an association of fossil forms identical with those 

 of South Devon. In 1839 I noticed the occurrence of certain similar forms in the rocks of the South of 

 Ireland ; and, guided by the identification of about forty species, I suggested, that in the Rhine and 

 Eifel countries (Paffrath, Bensberg, Gladbach, Gerolstein, &c.) there were equivalent deposits*. 



The not adopting, in this memoir, the name " Devonian" for these deposits, is not owing to ignorance 

 of its value as a geological group, but because such a name is at variance with the nomenclature of well- 



* Vide Report of British Association, 1839, p. 69. 



