12 Rev. J. F. Blake — Form of Sedimentary Deposits. 



EXPLANATION OF PLATE I. 



EiG. 1. — LicJias {Corycloceplialus) anglieics, var. wenloehensis. Outline restoration 



of pygidium. x 3. 

 ,, 2. — L. {Corydo.) angUcus,'VBx. obtusicaudatiis. OutKne restoration of pygidium. 



X 4. 

 ,, 3. — L. {Corydo.) hirsutus. Specimen with eleven thoracic segments, x 2J 



(partly restored). Woodw. Mus. 

 ,, 4. — Ditto. Specimen with eleven thoracic segments, x 2^ (partly restored). 



Brit. Mus. 

 . ,, 5. — Ditto. Outline restoration of pygidium from Fletcher's type, x 3. 

 ,, 6. — Ditto. Var. tuber culatiis. x 3. "Woodw. Mus. 

 ,, 7. — L. {Dicranopeltis) Woodivardi. x IJ. "Woodw. Mus. 

 ,, 8. — Ditto. Nat. size. Figured by Fletcher as L. Barrandii (Q.J.G.S., vol. vi, 



pi. xxvii, fig. 10). "Woodw. Mus. 



III. — On the Original Form of Sebimentary Deposits.^ 

 By the Rev. J. F. Blake, M.A., F.G.S. 



THE form of the deposits that are taking place on the sea-bottom 

 at the present day is one of the essential elements required 

 to be known when we wish to interpret the submarine contours, as 

 throwing light on the submergence or elevation of the land in late 

 geological times, or when we propose to use the variation of thickness 

 of the strata deposited during any epoch as an indication of the 

 position of the shore-lines at that time. 



In the case of dejDosits in small or temporary masses of water, 

 their form and arrangement may sometimes be observed directly ; 

 but in the case of the deposits in the sea, where we can neither 

 remove the water nor make borings beneath it, we can only avail 

 ourselves of theoretical considerations. 



It might have been expected that the original form of various 

 sedimentary deposits would have been considered in detail long ago, 

 but as a matter of fact the few writers who have touched upon the 

 question have mostly been content with the assumption that deposits 

 taken as a whole are thickest near the source of supply, and the 

 figures given in illustration of the arrangement of various kinds, and 

 thereby the shape of each, are remarkable for their variety.- 



As the theoretical results at which I have arrived differ funda- 

 mentally from the ordinary assumptions, it is to be hoped that 

 some one will be able to point out the fallacy, if any, which has led 

 me astray, and to explain more satisfactorily the observed features 

 which appear to confirm the theory. It will be seen, however, that 

 it is just those writers who have paid most attention to the matter 

 who approach most nearly to agreement with my results. 



The actual form of any deposit on the sea-bottom, supposed, for 

 the sake of argument, to be flat, will depend first upon the forces 

 to which the material is subject, and secondly upon the nature of the 



1 A paper read at the Meeting of the British Association, Belfast, September, 1902. 



2 See Godwin-Austen, Q.J.G.S., vol. vi, p. 82, fig. 2 (1850); Hull, Q.J.G.S., 

 vol. xviii, p. 135, fig. 4 (1862) ; Green, Lectures on Coal, p. 9 (1878), and Geology, 

 p. 211 (1882); Page & Lapworth, Introductory Textbook, p. 59, fig. 22 (1888); 

 Marr, Principles of Stratigraphical Geology, p. 117, fig. 13 (1898) ; "Watts, Geology 

 for Beginners, p. 73, fig. 47 (1898). 



