Oil the Genesis of Some Scottish Minerals. 187 



deposits sometimes replace Dolomite ; but more often dis- 

 place clays and shales. 



Dolomite, the third mineral in this list, has also been 

 formed by more than one method. Most of the Scottish 

 Dolomites are distinctly traceable to the alteration, in sit4, 

 of ordinary limestones — the change having been effected 

 chiefly, if not entirely, by the infiltration into the joints 

 and other divisional planes of the limestone, of solutions of 

 carbonate of magnesia derived directly or indirectly from 

 the New Eed Eocks. The vein-dolomites (Pearlspar), so 

 well seen at Wanlockhead and Leadhills, may possibly be 

 referable to downward percolation from the same source — 

 for there is reason for believing that the New Red Rocks ^ 

 (and therefore the Jurassic Rocks) formerly covered a large 

 part of Scotland. Nearly all the druses found in the 

 Scottish limestones of Carboniferous age are due to the 

 contraction in volume of limestone when it enters into 

 combination with carbonate of magnesia. The shrinkage 

 may amount to as much as one-twelfth of the whole (see 

 Heddle, Trans. Boy. Soc. Edin., xxvii.). Beds of Dolomite, 

 which can be shown to have been originally deposited as 

 such, are rare or absent entirely from Scotland. Various 

 compounds of the carbonates of iron, lime, and magnesia 

 occur in Scotland, but it will probably come to be generally 

 recognised that most of these are traceable to deposits from 

 infiltrations from the New Red. 



Some curious nodular and concretionary masses of car- 

 bonate of lime occur on at least two different creolos^ical 

 horizons in Scotland. Both of these fall to be noticed here. 

 The older of these forms the Cornstone Series, which marks 

 the transition from the Upper Old Red Sandstone to the 

 Ballagan Beds or Lower Limestone Shale. It is well seen 

 in the sedimentary rocks of Salisbury Crags. It usually 

 occurs in the form of irregular nodules ; but may graduate 

 into mere flakes on the one hand, or into irregular and im- 

 persistent interstitial deposits within the sandstones. These 



^ This is employed here as a convenient general name for any of the Red 

 Rocks which are intermediate in age between the Carboniferous and the 

 Rhsetic Rocks. A term having that meaning is very much needed. 



