296 W. 0. Crosby — On the Appearance of a Fault. 



EXPLANATION OF PLATE VII. 



Fig 1. — a. Two examples of Cladochonus (Aulopora ?) Michelini, E. and H., of the 

 natural size, Lower Carboniferous, Dunbar. 



b. A small example of the same enlarged five times. 



c. A longitudinal section supposed to be of the same species, enlarged five times. 



d. Portion of a colony of Aulopora sp., from the Devonian of Ontario, of the 



natural size. 



e. Longitudinal section of part of the same, enlarged five times. 



f. Cross-section of a corallite of the same, similarly enlarged, showing the tabulae. 



g. Portion of a colony of Aulopora repens, E and H., from the Eifel, of the nat. size. 

 /;. Section of the same, enlarged seven times, showing curved tabula?. 



Fig. 2. — a. A full-grown colony of Monilopora crassa, M'Coy, growing upon the 

 stem of a crinoid, of the natural size. 



b. A younger colony of the same, encircling a crinoidal column, and viewed from 



above, of the natural size. 



c. A detached fragment of the corallum of the same, of the natural size. 



d. Transverse section of a young colony of the same growing upon a crinoidal 



column, magnified 2± diameters (the visceral cavities of the corallites are more 

 or less largely filled with matrix, and the peculiar reticulated structure of the 

 skeleton is here and there visible in the wall, while the whole has been finally 

 enveloped by the growth of the stem of the crinoid). 



e. Longitudinal section of a single corallite of the same, enlarged five diameters, 



showing the open visceral chamber, the fibrous wall, and the reticulated structure 



of the wall of the calice. 

 /. A portion of the reticulated tissue still further enlarged. 

 All the specimens are from the Carboniferous Limestone of Lancashire. (British 



Museum.) 



II. — How the Appearance of a Fault mat be Produced 



without Fracture. 

 By W. 0. Crosby, S.B. 



THE general structure and mode of growth of coral reefs ana- 

 islands is well understood ; but there is one attendant circum- 

 stance of considerable geological interest, to which, so far as I am 

 aware, attention has never been called. This is the peculiar strati- 

 graphic relation of the different strata in the reef to their chrono- 

 logical equivalents in the deposits of the surrounding ocean ; a 

 relation due to the comparatively rapid growth of the reef as a whole. 

 Deposits of some sort, it may be safely said, are forming in all 

 parts of the sea, of coarse materials and with comparative rapidity 

 in shallow portions adjacent to the land, and of finer sediments and 

 with extreme slowness in the oceanic abysses remote from the conti- 

 nental borders. Calcareous sediments usually accumulate much more 

 slowly than those of mechanical origin, but to this statement the 

 limestones of coral reefs probably constitute an important exception ; 

 while the growth of the reef under favourable circumstances is in- 

 comparably more rapid than the vertical increase of the impalpably 

 fine, and mainly organic, ooze and mud covering the greater part of 

 the ocean floor. This is proved by the fact that the coral reef and 

 island are usually able to keep pace during countless ages with the 

 progressive subsidence of the sea-bottom, so that their living 

 growing crests are always within reach of the sunlight and air, 

 while the dead and consolidated mass below stands like a wall 

 towering thousands of feet above the slowly increasing and yet strictly 

 synchronous deposits of the surrounding ocean, which appear to main- 

 tain their fineness and uniformity up to the very base of the reef. 



