INTRODUCTION. xv 



centre (repeatedly pullulating similar bodies in their progress from all its sides), where they were the most 

 numerously developed, regardless of the latter answering to the plane of a cleavage split, or of a deposition 

 parting: the interspaces between the branches of the coralloidal bodies were filled with a yellow ochreous 

 magnesio-calcareous powder; while the branches themselves were radiately crystallised, and essentially 

 composed of carbonate of lime. Since the period referred to, I have seen similar phenomena in other 

 places J as in the railway cutting between Sunderland and Ryhope, where the beds are distinctly divided 

 by cleavage splits, from the walls of which, and also from those of the deposition partings, innumerable 

 crystalline bodies, resembling the Music Pipe Coral, branch off in the most striking manner, giving the 

 beds the appearance as if they were formed of vast petrified coral reefs. At Fulwell and Southwick quai-ries, 

 where the beds have not been so decidedly acted on by cleavage forces, I have seen enormous stalactiticoid 

 and maramillated masses shooting off, both upwards and downwards, base to base, from the plane of 

 horizontal deposition partings, — the law of gravity having been altogether disregarded by the mysterious 

 agent of their production. It required no effort of the mind to perceive, that the coralloidal and mammillated 

 forms had been produced by one and the same agent, and that this agent had thus operated since the depo- 

 sition and consolidation of the rock, nay, even subsequently to the completion of the cleavage structure. 

 Other inferences, as the following, spontaneously flow from a right interpretation of the phenomena observed : 

 1st, the entire rock was originally magnesio-calcareous in its composition ; 2d, the ochreous portions have 

 not undergone any chemical or molecular change ; 3d, the rock became divested of the magnesia when 

 it assumed the crystalline form; -Ith, the crystalline portions have undergone a complete chemical and 

 molecular change. 



But it may be asked — ^how, or in what manner, has this change — this metamorphism been produced? 

 This is a question which I can only answer hypothetically. Apparently several circumstances have con- 

 tributed to the change; namely, the chemical composition of the rock, cleavage splits, and deposition 

 partings ; for it is impossible to ward off the idea, that they are in some way or other connected with the 

 coralloidal structures. The cleavage splits naturally direct our attention to their producing agent ; but an 

 important point requires to be settled before it can be made available in the present inquiry ; — thus, is 

 cleavage a mechanically or a physically induced structure 1 If the formei", I should not be in the least 

 disposed to invoke its aid ; but if the latter, a powerful agent seems to be at our command. The cleavage 

 agent, for the reason above stated, however, does not avail us in working out the problem under con- 

 sideration. As a last resort, — no answer appears so satisfactory as the one suggested by the hypothesis, that 

 some volatilising agent, in passing or circulating through the cleavage splits and deposition partings, has 

 acted here and there on the original constituents of the rock, expelling the magnesia in such places, and 

 crystallising the residual carbonate of lime ; while the former constituent would remain associated with the 

 calcareous base wherever the rock was not acted on. I am certainly more inclined to adopt this \'iew than 

 the one which maintains the change to have been produced by simple chemical segregation when the rock 

 was in a soft state, as proposed by Professor Sedgwick ; or that which contends for the partial permeation 

 of the beds at any indeBnite time after their formation, either by vapours of some form of magnesia evolved 

 from subjacent igneous sources,'^ or by a magnesian solution effused from overlying oceanic reservoirs, as 

 proposed by some other writers. 



It may be objected, that this hypothesis does not explain the origin of the globulo-concretionary masses, 

 which are often seen where the coralloidal structures are not developed. If these masses were decidedly 

 isolated (iq some cases the connexion between them is very slight), and in the heart of a pulverulent bed, 

 without any connexion with the deposition partings, or cleavage splits, I admit that there would be some 

 reason for withholding assent to it ; but, as neither one, nor the other of these conditions prevails, this 

 objection readily falls to the ground, whether it be based on what is displayed in the bed of " cannon-balls" 



1 The Sunderland Magnesian limestone has often been termed Dolomite ; but until it is certain, that the 

 magnesia of this deposit has originated in the same way as that of the true Dolomites of the Tyrol, it seems 

 premature to make such a use of this mineralogical terra. Respecting the origin of the magnesia contained 



