SOME MBTASOMATIC CHANGES IN LIMESTONES. 57 



the coarsely crystalline modification near the contact with 

 granitic intrusions is in general non-magnesian. It is per- 

 haps more questionable whether such an idea can be applied 

 to the Durness limestones of the Assynt district in Suther- 

 land, which, according to analyses given by Heddle, are in 

 great part perfectly dolomitised, while the Ledbeg marble, 

 which is their metamorphosed equivalent near the large in- 

 trusion of Borolan, is much less magnesian. In these and 

 similar groups of limestones, the facts that certain zones or 

 beds are described as dolomitic, and others alternating: with 

 them as non-dolomitic, might seem to suggest that the 

 magnesian nature of the former was closely connected with 

 the accumulation of the deposits. Such an argument, how- 

 ever, is by no means conclusive ; we may with equal reason 

 suppose that at certain horizons in the series of deposits 

 there was something: in the character of the rocks which 

 predisposed them to the transformation to dolomite. At 

 least the manner in which one and the same bed may 

 sometimes be seen to pass abruptly from a normal lime- 

 stone to one composed two-thirds of dolomite proves that, 

 in such a case, the magnesian character has been acquired 

 by some secondary change distinctly posterior to deposition. 



The well-known concretions of crystalline calcite in some 

 beds of the magnesian limestone of Durham also suggest 

 themselves. Garwood (8) has shown that the ratio 

 MgCO, : CaC0 3 , which in the matrix of these bodies is 

 6674 : 100, is in the concretions themselves only 11 : 100. 

 In his view the formation of the concretions is posterior to 

 the dolomitisation, and has been influenced by the amount 

 ol magnesia in the several beds, either too high or too low 

 a proportion checking or preventing the development ol 

 the structure. The facts, as stated by him, do not, however, 

 seem to be inconsistent with the alternative supposition that 

 the already existing crystalline concretions have resisted the 

 dolomitising process by which the rest of the rock has been 

 transformed. 



Leaving these doubtful questions we may pass on to a 

 few remarks on another common metasomatic change, or 

 series of changes, in limestones ; that which results in the 



