438 E. J. Garwood— -Origin of the Concretions 



being lined witli a similar deposit. Figs. 3 and 4, on Plate XIL, show 

 concretions coated with it. In many cases (Fig. 4) the calcite has begun 

 to replace the original concretion, or perhaps add to its original mass, 

 but this replacement is limited to about a quarter of an inch on the 

 surface, and it never penetrates far, except when it fills cracks in 

 the concretion. It is obvious, then, that it is either replacing the 

 concretion from without inwards, or is forming on the surface of 

 previously existing concretions, and its secondary nature is quite 

 unmistakeable. 



Summary. — We can then sum up the evidence against the stalactitic 

 origin of these concretions under the following heads : — 



I. StratigrapMcal. — (a) The marked stratified condition of these 

 beds as a whole, which would to a large extent have been obliterated 

 by the copious infiltration necessary for the formation of the concre- 

 tionary structures. 



(b) The uninterrupted passage of the lines of bedding through, 

 matrix and concretions alike. 



(c) The enormous thickness of some of the concretionary limestones. 



(d) The fact that the concretions occur only in the Upper Magnesian 

 Limestone, which is overlaid by the Triassic Marls and Sandstones, 

 a deposit containing originally probably but little carbonate of lime. 



(e) The fact that the concretions are confined to the Magnesian 

 Limestone series. 



II. Evidence from fossils. — (a) The occurrence of shells in the 

 centre of the radial concretions. 



(b) The occurrence of cavities at the centre, probably originally 

 occupied by organic matter. 



(c) The wonderful state of preservation of shells occurring in these 

 concretions and the almost total absence of fossils from the earthy 

 matrix. 



III. Chemical. — (a) The great similarity in composition between 

 the concretionary bed as a whole, and the massive beds higher in the 

 series which contain no concretions, with reference to the relative 

 amounts of carbonate of magnesia and carbonate of lime that they 

 contain. Showing that it is unnecessary to assume the introduction 

 of extra carbonate of lime from beds above. 



(&) The variable amount of magnesia in contiguous concretions. 



(c) The presence in the concretions of a considerable quantity of 

 insoluble material. 



(d) The presence of calcite in these beds which has undoubtedly 

 been deposited by stalactitic action and which in appearance, com- 

 position, and behaviour is so dissimilar to these concretions. 



Having shown that the carbonate of lime of the concretions has 

 in all probability been derived from the beds in which the concre- 

 tions occur, we should like to add a few words as to the manner in 

 which they appear to have been produced. 



That some of the carbonate of lime disseminated through the 

 deposit was abstracted from the general mass, and segregated round 

 definite centres of attraction, is doubtless true, and, as Prof. Green 

 remarks, we may give to this process a name of its own and call it 



