246 L. M. Parsons — Dolomitization 



at present too incomplete to allow any conclusions to be formed as to 

 the genesis of the ores. 



Theoretical discussion of the facts set forth in this section will be 

 postponed until after the scheelite deposits have been described, 

 since in many instances the problems involved are very similar. 

 (To be continued.) 



II. — Dolomitization AND the Leicestekshiee Dolomites. 



By L. M. Parsons, M.Sc. (Lond.), D.I.C., F.G.S. 



(PLATE XI.) 



Part I : Evidences of the Period of Dolomitization. 



Contents. 



1. Classification of Types. 



2. Field Evidences. 



3. Inherent Structural Evidences. 



4. Selective Dolomitization. 



5. The Absence or Presence of Fossils. 



6. Classification of Petrological Types. 



7. The Dense Yellow Dolomites of Breedon, etc. 



8. The Bed Ferruginous Dolomites of Breedon and Breedon Cloud. 



9. The Barren Grey and Yellow Dolomites of Ticknall and Calke. 

 10. Fossiliferous Dolomitic Limestones of Ticknall and Calke. 



IN the district north of A.shby-de-la-Zouch doloraitized Carboniferous 

 Limestone crops out where the Leicestershire border adjoins that 

 of South Derbyshire. The dolomites, which attain a thickness of 

 nearly 900 feet in this area, have hitherto received very little attention. 

 I therefore propose to give a short description of them with a view 

 to ascertaining how far their mode of occurrence and structure afford 

 additional examples of , or exceptions to, the usual conclusions adoptei 

 concerning the origin of dolomite. With this object in view I propos'^ 

 to give a brief resume of the evidences generally relied upon to 

 explain the origin of dolomite, before proceeding to describe the 

 Leicestershire rocks. 



1. Classification of Types. 

 If dolomites are classified .as simply as possible according to the 

 period at which the dolomitization ^ took place, well-defined classes, 

 as enumerated below, may be recognized: — 



( 1. Those deposited as clastio rocks derived from 

 Primary I pre-existing dolomite. 



dolomites, ] 2. Those chemically precipitated as dolomite, with 

 or without the agency of organisms. 



1. Contemporaneotis dolomites, or those deposited 

 as ordinary limestones which have been altered, soon 

 after deposition, by the influence of magnesian salts in 

 the sea in which the rocks were originally deposited. 



2. Subsequent dolomites deposited as ordinary 

 limestones which have been altered by the influence 

 of waters belonging to some period later than that 



, during which the rocks were originally deposited. 



^ Throughout this article the term dolomitization signifies the production of 

 dolomite, either primary or secondary. 



Secondary 

 dolomites. 





