134 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[Feb. 5, 



Fig. 3. — Showing the Distribution of the Calcareous and "Sedimen- 

 tary^' Strata of the Great Oolite. 



North Yorkshire. 



South 

 Oxfordshire. 



Sandstone ) 

 and shale. S 



Regarding, then, the calcareous strata as differing in their origin 

 and in their mode of distribution from the other stratified rocks with 

 which they are associated, it appears to me that it is incorrect to 

 class them together under the same term of " sedimentary." I there- 

 fore propose to eliminate the limestones from this category, and to 

 place them as a distinct class of rocks, confining the term " sedi- 

 mentary " to gravels, sandstones, shales, and clays, with their vari- 

 ations. The presence of each class of rock in the same geological 

 group is no argument in favour of their similarity. Whenever inter- 

 stratifications occur, they may be regarded as occupying the neu- 

 tral ground between their respective areas of dispersion ; and I have 

 little doubt, could it be possible to trace the sources of the " sedi- 

 mentary " strata of any formation on the one hand, and of the lime- 

 stones on the other, they would be found expanding in opposite di- 

 rections, and, as it were, originating at opposite poles. The relation- 

 ship here contended for will be rendered more clear in the case of 

 the British Carboniferous Rocks by the iso-diametric lines presently 

 to be described. 



(h.) Cause of the frequent occurrence of a Threefold Arrangement in 

 Groups of RocTcs, with a central Calcareous Member. — We cannot fail 

 to have observed that many groups have a tendency to arrange them- 

 selves into threefold divisions, the upper and lower being composed 

 of sands or clays, the middle of limestone. This has been remarked 

 as the result of his observations on the continent by Sir R. Murchi- 

 son, and we have many examples in this country. Thus in the 

 Upper Silurian Rocks there is a calcareous centre. This is also the 

 case in the Devonian group of Devon and Cornwall ; in the Carboni- 

 ferous, the Permian, the Triassic (when complete), and the Jurassic 

 Rocks. 



Phenomena of so general a character cannot be accidental, but 

 must be in accordance with the system of nature. May not the fol»- 

 lowing be the true explanation ? 



believe, in accordance with the order of succession determined by Prof. Phillips. 

 Notwithstanding, howerer, the existence in the Scarborough Limestone of an 

 Ammonite which is characteristic of the Inferior Oolite in Gloucestershire, I am 

 strongly inclined to believe, on stratigraphical grounds, that there has been a 

 " migration" of species ; and that this is a case where identity of fossil remains 

 does not prove that the strata are contemporaneous. In either case, however, 

 the illustration is of equal value for my purpose. 



