1863.] DAY LIAS OF DORSETSHIRE. 279 



years ago by the late Sir Henry De la Beche*. When I undertook 

 the investigation of the geology of the coast in the neighbourhood 

 of Lyme Regis, I soon found that that section, though according 

 well with certain general facts, was, owing to the rapid progress of 

 geological knowledge, and the consequent changes in our system of 

 classification, utterly useless to students. The divisions and sub- 

 divisions of De la Beche were founded solely on lithological grounds, 

 whereas those of the present day are based in great measure upon 

 paheontological considerations. He classed all our Upper and nearly 

 half our Middle Lias with the Inferior Oolite Sands f, and he sub- 

 divided the Lias, as then understood, accordingly. As he had classed 

 sands with sands, and marls with the Lias, so he divided the latter 

 into " Upper Lias Marls " (which, under the present system, include 

 both the Middle and Lower Lias Marls), " Lias Limestones " (which 

 correspond to a subdivision of the Lower Lias), and " Lower Lias 

 Marls" (which are the "Avieula contorta series," no longer regarded 

 even as Liassic). 



The section quoted gave to the entire Lias Formation of Dorset- 

 shire a thickness of 600 feet ; our present acceptation of the term 

 includes between 1000 and 1100 feet of strata. 



The plan which I have adopted, in order to lay clearly before the 

 Society the extent and character of these deposits, is to give, in the 

 first place, a description of them lithologically as they appear in the 

 successive cliffs ; and, in the second, to point out, in a generalized 

 vertical section, the palaeontological features which characterize the 

 subdivisions and strata. To avoid confusion, I will premise what I 

 understand by the terms Middle and Upper Lias. 



2. The Middle Lias. — As this term is now accepted, it may be 

 defined as that portion of the Lias Formation which lies between the 

 strata containing Ammonites raricostatus, Ziet., as the predominant 

 form below, and those characterized by Ammonites communis, Sow., 

 above. 



Thus denned, the Middle Lias of this neighbourhood presents a 

 mass of beds which, though including two great and lithologically 

 well-marked subdivisions, is on the whole very distinct, both in its 

 lithological appearance and in most of its fossils, from the beds above 

 and below. The Marlstone and the Middle Lias Sands are generally 

 understood to occupy this interval, or, at least, the greater part of it, 

 throughout the South-west of England ; but, in the district under 

 consideration, these beds do not occupy more than two-fifths of the 

 gap, the larger part being filled up by a mass of marls. Throughout 

 the rest of England, moreover, I believe the total thickness of this 

 member of the Lias amounts to much less than in this district, where 

 it includes upwards of 500 feet of strata. 



Such an increased development, and other characters very much 



* G-eol. Trans. 2nd series, vol.ii. 1829; also, Eeport on the Geology of Corn- 

 wall. Devon, and West Somerset, 1839. 



t Owing to this classification, fossi.s from the Middle Lias hare been given 

 as from the Inferior Oolite of Bridport (vide Bhynchonella serrata, Pentaerinus 

 gracilis, &c. in Morris's Catalogue of British Fossils). 



u 2 



