40 Prof. How on the Mineralogy of Nova Scotia. 



sometimes to originate indirectly from anhydrite by absorption 

 of water. Here we must remember, as Von Cotta says, that " the 

 supposed origin of gypsum from anhydrite leaves the greater 

 difficulty unsolved of the original deposit of anhydrous sulphate 

 of lime " * ; and this rock we have here containing imbedded in 

 it hydrated minerals, namely selenite and silicoborocalcite. The 

 latter being in rounded nodules, may have been reduced to that 

 form before being included ; but the angular, lustrous, and trans- 

 parent crystals of selenite cannot have been subject to action 

 capable of so affecting a body originally angular as to render it 

 a pebble. The nodules of hard silicoborocalcite are imbedded (so 

 far as I observed — and I examined the accessible parts of some 

 300 tons of quarried rock piled in a low heap for shipment, and 

 also saw the mineral in situ) exclusively in anhydrite, the soft 

 exclusively in gypsum ; there is an intermediate degree of hard- 

 ness in the mineral found in a matrix composed of both these 

 rocks. We might hence conclude that the soft results from the 

 hard borate in consequence of physical changes accompanying 

 the passage of anhydrite into gypsum : this is not impossible in 

 some cases ; but the gypsum holding the soft borate most abun- 

 dantly is not only so much less pure a rock than the anhydrite 

 holding the hard nodules that it could not have arisen by mere 

 absorption of water, but there are frequently imbedded in it 

 separate nodules of natroborocalcite, which I have never seen in 

 anhydrite. As regards the passing of anhydrite into gypsum, 

 what proofs are there of its ever occurring ? Here we see alter- 

 nations of these rocks below the surface : at Windsor, for exam- 

 ple, large lenticular masses of anhydrite, from 2 to 10 feet thick 

 in the centre and some 50 feet long, lie in the midst of gypsum 

 brought to view by quarrying. In other places there are lofty 

 cliffs composed largely of anhydrite on their surface : at Cape 

 Canseau, for instance, I am told by Professor Lawson, the bluff, 

 exposed to the wash of the ocean as well as the action of the 

 atmosphere, is anhydrite, not gypsum. From a consideration of 

 these circumstances, it appears to me that in sedimentary rocks 

 even, where gypsum might be derived from anhydrite, but the 

 converse is not probable, these minerals must sometimes have an 

 independent origin. In the present case I think it must be so, 

 because of the exclusive occurrence of the hard nodules of silico- 

 borocalcite in anhydrite, and from the absence of natroborocalcite 

 from this rock, while it occurs abundantly in the impure gyp- 

 sum holding the soft silicoborocalcite with selenite. However 

 these rocks and their borates may have originated, it is clear 

 that as deposits they were contemporaneous. 



* Rocks Classified, p. 292. 



