254 L. M. Parsons — Dolomitization 



In this table the types predominating in the district are placed 

 higher in the list. 



7. The Dense Yellow Dolomites of Beeedon, Beeedon Cloud, etc. 



The bulk of the dolomites of the two Breedons, Barrow Hill, and 

 Osgathorpe consists of dense yellow material having a specific gravity 

 and chemical composition approaching those of a pure dolomite. 

 The proportion of Magnesium Carbonate varies slightly in different 

 beds, but averages nearly 40 per cent, while iron compounds and 

 insoluble residues are present in small amounts. 



The field relations of these rocks are studied best at Breedon-on- 

 the-Hill, where quarries are being worked in a direction at right 

 angles to the strike. At this locality more than 800 feet of fairly 

 thick-bedded dolomites succeed one another without any marked 

 variation in petrological characters and without any apparent com- 

 plications due to faulting. Though the chemical composition and 

 texture of the material forming one stratum may be slightly different 

 from that of another, the inherent characters of any particular bed 

 appear to be uniform. The dolomitization is in no case patchy (/).^ 

 Laterally the beds do not pass into unaltered or poorly dolomitic 

 limestones (d), though this fact would have greater significance if the 

 Carboniferous Limestone of the area had a larger outcrop. The 

 absence of faulting at Breedon suggests the improbability of 

 subsequent vein dolomitization associated with dislocation (y). At 

 Breedon Cloud strike faulting does occur, but is there associated 

 with non-patchy yellow dolomites which do not pass laterally into 

 unaltered limestone (h). 



Conglomerates and pseudo-breccias are not present. Microscopic 

 sections of the yellow dolomite of Breedon, Breedon Cloud, and 

 Barrow Hill show a fine-grained crystalline structure composed 

 mainly of small grains more or less allotriomorphic, though some 

 rhombohedral outlines may be seen {j and t). The degree of purity 

 is not high since minute dusky inclusions of insoluble matter are 

 very numerous {k). There are no zonal or central inclusions of 

 haematite, and what little iron oxide does occur, mainly limonite, is 

 interstitial (/). Chert is absent from the material exposed in the 

 workings of Breedon-on-the-Hill, though it occurs in the yellow 

 dolomites of a higher horizon at Breedon Cloud. Sections do not 

 show any dolomite rhombohedra in the chert (wi). Eossils are not 

 numerous at Breedon, Barrow Hill, and Osgathorpe; those that do 

 occur at these localities consist chiefly of dolomite casts of Brachiopoda 

 and a few corals. At Breedon Cloud, however, higher beds are 

 exposed and fossils are more plentiful. Corals are preserved as 

 dolomite replacements and as casts (r). Syringopm-a is usually 

 found as casts, but Michelitiia, Campophyllum, and other genera exhibit 

 septa, tabulse, and other structures beautifully preserved in minutely 

 crystalline dolomite. Goniatites ( GlypMoceras) also occur as dolomite 

 replacements, but Brachiopoda are present as casts. 



The conclusion to be drawn from the collective evidence concerning 



^ Italic letters in parentheses refer to corresponding evidences mentioned in 

 the earlier part of the article. 



