434 E. J. Garwood — Origin of the Concretions 



is at Marsden quarries on the coast of Durham, between Sunderland 

 and South Shields, where masses of stratified limestone are inter- 

 bedded with marls containing concretions in every stage of de- 

 velopment. Where fully developed, the concretions assume a 

 spherical form, and are sometimes as much . as 1 to 2 feet in 

 diameter, the average size being from 3 to 6 inches. They are 

 composed of fibrous crystals of calcite radiating symmetrically 

 from the centre, which frequently consists of a valve of Ascimis 

 dubius or Myalina Hausmanni. (See Plate XII. Fig. 1.) Very 

 often, however, the centre is composed merely of a small cavity. 



These concretions occur in irregular masses 20 to 30 feet thick, 

 showing here and there rough stratification, and often passing 

 laterally into well-bedded limestone. Many of the larger specimens 

 show well-marked concentric bands developed at regular intervals. 



The concretions are piled on one another with but scant matrix 

 between ; chemically they consist of 91 to 95 per cent, of carbonate 

 of lime and 1-5 to 4 per cent, of magnesia. 



It is difficult to see how such masses of limestone could have been 

 formed by infiltration from beds above. Where are the beds into 

 which the carbonate of lime dripped during the formation of the 

 concretions ? If we suppose these concretions to have been formed 

 from carbonate of lime introduced in solution from an overlying 

 bed subsequent to the consolidation of the deposit in which they 

 occur, then the original bed, now replaced by concretions, must 

 have disappeared. But this bed would originally contain 20 to 30 

 per cent, of magnesia, which must have been subsequently dissolved 

 out ; yet it is hardly likely that water which was saturated with 

 carbonate of lime would dissolve out carbonate of magnesia the less 

 soluble salt of the two,^ and we cannot suppose that the magnesia bed 

 was dissolved out first, leaving an enormous cavity into which the 

 carbonate of lime afterwards dripped, forming around shells sus- 

 pended in some mysterious manner ; the symmetrical development 

 of the concretions also, which is as perfect vertically upwards from 

 the nucleus as it is laterally and vertically downwards, seems to 

 preclude the possibility of their having been formed in a previously- 

 solidified rock-mass. 



Again, the shells forming the nucleus of the concretions are often 

 in beautiful preservation, which is hardly likely to have been the 

 case if they had lain in a porous bed exposed to the action of 

 infiltrating waters, while in those cases where the centres are hollow 

 it is impossible for the carbonate of lime trickling through the 

 beds in solution to have crystallized out round a cavity, whereas we 

 can easily imagine the centre to have originally consisted of an 

 organic body in the sediment which, serving as the nucleus round 

 which concretionary action was set up, afterwards decomposed. 



But it is in the beds containing immature concretions in various 

 stages of growth that the strongest arguments against the formation 

 of the concretions by stalactitic action are supplied. 



1 Hardman, Carbonif. Dol. of Ireland, Proc. Eoy. Irish Acad. vol. ii. ser. 2, 1877. 



