310 ORIGIN OF CRYSTALLINE ROCKS. [XIII. 



niagnesian carbonates in the crystalline schists is explained, as 

 marking portions of one continuous process. The formation of 

 these mineral waters depends upon the decomposition of feld- 

 spathic rocks by subterranean or sub-aerial processes, which were 

 doubtless more active in former ages than in our own. The 

 subsequent action upon magnesian waters of these bicarbonated 

 solutions, whether alkaline or not, is dependent upon climatic 

 conditions ; since, in a region where the rain-fall is abundant, 

 such waters would find their way down the river-courses to the 

 open sea, where the excess of dissolved sulphate of lime would 

 prevent the deposition of magnesian carbonate. It is in dry 

 and desert regions, with closed lake or sea basins, that we must 

 seek for the production of magnesian carbonates; and I have 

 argued from these considerations that much of northeastern 

 America, including the present basins of the Upper Mississippi, 

 Ohio, and St. Lawrence, must, during long 'intervals in the 

 palaeozoic period, have had a climate of excessive dryness, and 

 a surface marked by shallow enclosed basins, as is shown by the 

 widely spread magnesian Limestones, and by the existence of 

 gypsum and rock-salt at more than one geological horizon within 

 that area.* (Ante, page 76.) The occurrence of serpentine and" 

 diallage at Syracuse, New York, offers a curious example of the 

 local development of crystalline magnesian silicates in Silurian 

 dolomitic strata, under conditions which are imperfectly known, 

 and, in the present state of the locality, cannot be studied, t 



Since the uncombined and hydrated magnesian mono-carbon- 

 ate is at once decomposed by sulphate or chloride of calcium, it 

 follows that the whole of these lime-salts in a sea-basin must be 

 converted into carbonates before the production of carbonated 

 magnesian sediments can begin. The carbonate of lime formed 

 by the action of carbonates of magnesia and soda remains at 

 first dissolved, either as carbonate (ante, page 140) or as bicar- 

 bonate, and is only separated in a solid form, when in excess, 



* Geology of Southwestern Ontario, American Journal of Science (2), 

 XLVII. 355. 



t Geology of the Third District of New York, 108-110; and Hunt on 

 Ophiolites, American Journal of Science (2), XXVI. 236. 



