of Edinburgh , Session 1881-82. 
609 
Both on stratigraphical and chemical grounds, the conversion of 
dolomitic limestone into serpentine seems probable, but, so far as we 
are aware, no one has as yet pointed out the precise nature of the 
reactions involved in this change. 
Carbonate of magnesia decomposes at a much lower temperature 
than carbonate of lime, the difference between the decomposing 
points being so great that it is practically made use of in 
Pattinson’s process for the preparation of magnesia from the 
magnesian limestone of Durham. The limestone is calcined in close 
iron vessels at a dull red heat, the carbonate of magnesia alone 
undergoing decomposition at this temperature ( Pharmaceutical 
Journal t 1844, iii. 424). blow it is easy to understand that if a 
rock, such as the Campsie limestone, containing carbonate of lime, 
carbonate of magnesia and silica, were heated to a temperature 
sufficient to decompose the carbonate of magnesia, but not the 
carbonate of lime, the magnesia at the moment of liberation would 
combine with the silica, forming silicate of magnesia. Adopting 
the theory of hydrothermal action, the carbonic acid set free from 
the magnesia would be taken up by water, and would act as a 
solvent upon the carbonate of lime, removing it, and leaving behind 
hydrated silicate of magnesia only. 
That the carbonate of lime has not been removed from the 
specimens of the Campsie limestone which had passed through the 
kiln, is simply due to the fact that there was no water present to 
take up the carbonic acid. We do not mean, of course, to assert 
that in every instance this has been the character of the change, 
whereby dolomite has been altered to serpentine, but in the case 
of impure dolomites containing sand, the explanation is more 
satisfactory than Sterry Hunt’s somewhat vague theory of the 
action of heated solutions of alkaline silicates. 
The theory which we have briefly sketched seems to reconcile 
satisfactorily many of the facts connected with the composition and 
occurrence of serpentine. It explains — 
(1) The peculiar composition of such serpentines as those of 
Canada, and the frequent occurrence of carbonate of lime and car- 
bonate of magnesia in the serpentine of other localities (Zirkel, 
i. 324). 
