in the Magnesian Limestone. 437 



In order to compare the composition of this bed with that of our 

 bed of concretions, we must obtain the average composition of the 

 latter, and to do this we must determine the relative proportions of 

 concretions and matrix present. Numerous estimates along the 

 exposure give roughly 2 of matrix to 1 of concretions as the approxi- 

 mate proportions, and working this out (Column III. of the Table), 

 it gives us 40 parts of magnesia to every 100 parts of carbonate of 

 lime, which will be found to agree very closely with the analysis 

 of the Eoker bed, which gives 43 parts of magnesia to 100 of 

 carbonate of lime. 



This entirely confirms the theory that the lime composing the 

 concretions was derived from the beds in which they occur, and was 

 not introduced subsequent to their deposition. We must, however, 

 frankly acknowledge that there are two uncertain factors in this 

 comparison, namely, the difficulty of arriving at an exact estimate 

 of the relative proportions of matrix and concretions, and the some- 

 what arbitrary selection of the bed with which they are compared ; 

 as, however, great importance has been attached to a detailed 

 chemical investigation of these beds, we give the results of our 

 work in this direction, which confirm, as far as they go, what we 

 have before alluded to as the ' Segregation ' theory. 



There is, however, other evidence in this direction derived from 

 a study of the chemical composition of the concretions, namely, the 

 presence in them of magnesia and insoluble matter. 



The amount of magnesia, which is sometimes as much as 12 per 

 cent., varies considerably in different concretions in the same bed, 

 analyses of two concretions lying a few feet apart yielding in one 

 case 2-3, and in the other 6-3 per cent. If the concretions had 

 been deposited from percolating water, it is difficult to see why the 

 water should vary at the distance of a few feet in the amount of 

 magnesia it contained, whereas on the segregation theory we should 

 expect the amount of impurities imprisoned during the process to 

 vary from point to point, and it is still more difficult to account for 

 the presence of insoluble matter, which is usually about 2 per cent., 

 and in one specimen amounts to as much as 8-5 per cent, of the 

 concretion. 



The last argument against the stalactitic origin of this deposit 

 that we have to bring forward is the fact that large quantities of 

 calcite occur in these beds which liave undoubtedly been formed by 

 stalactitic action. They consist of pure white fibrous calcite," and 

 contain no magnesia or insoluble matter, they line cavities and 

 cracks, and fill in the interstices between the concretions, often 

 coating their surfaces to the thickness of an inch, and roughly 

 modelling themselves on their external shape. These deposits, which 

 here and there attain a thickness of 5 to 6 inches, can be traced to 

 joints in the overlying beds through which the carbonate of lime 

 has undoubtedly obtained access in solution, the walls of the joints 



1 Browell and Kirkby, on "The Magnesian Limestone of Durtam," Nat. Hist. 

 Trans, of Northumberland and Durham, vol. i. pt. 2. 

 3 Not aragonite, as often described locally. 



