30 WILMORE : THE STRUCTURE OF SOME CRAVEN LIMESTONES. 
branch of the subject. On the nature and origin of hmestones 
generally, the following literature has been of use in studying 
the examples in this district : — 
(1) The classic paper of Dr. Sorby (Q.J.G.S., 1879), " On 
the origin of certain limestones." 
(2) The work of Sollas on Coral Reefs, Derived Lime- 
stoaes, &c. ; various papers. 
(3) Barker's Petrology. Geikie's Text Book, 4th edition. 
(4) Rosenbusch, Elemente der Gesteinslehre, and the 
excellent account and discussion in Zirkel's Lehrbuch 
der Petrogrophie, vol. iii., under the heading " Bildun:^ 
der Kalksteine." 
The Nature of the Limestone Rocks. 
{a) The white or greyish-white limestones. 
It is, of coarse, well known to everyone that much of the 
limestone rock of this Craven Lowland country evidently consists 
of organisms ; shells (Mollusca and Brachiopoda), Polyzoa, 
Echinodermata (Crinoidea and Echinoidea, and Blastoidea), and 
corals. The knoll masses of Clitheroe and Downham have 
long been a favourite collecting ground, and the shells and other 
fossils may often be extracted in great perfection. The limestone 
masses thus built up — as they often are — almost exclusively 
of shells are usually of the light grey (so-called white) type ; 
but these shell masses are not by any means confined to the 
detached or semi-detached masses known as knolls on the one 
hand, nor to the white limestone on the other. 
The famous Bold Venture Quarries, near Chatburn Station, 
have long yielded abundance of the most perfect fossils — Brachio* 
poda and Mollusca (Gasteropoda, Cephalopoda, and Lamelli- 
branchiata) — but these beds are quite half a mile from the 
southern edge of the beds w^here the knolls occur, and are thus 
(taking the dip as not more than 30°) at least 1,300-1,400 feet 
down in the series. There are other exposures in the Chtheroe- 
Chatburn area which are also very fossiliferous. 
Again, shell aggregates are not confined to the white type 
of hmestone. At West Mart on there is a dark limestone, with 
