Dolomitication in the CarhoTiiferous Limestone. 57 



thick, in which, bedding planes are often traced with difficulty. 

 The material on the same horizons in the main dale at Gratton and 

 around Jughole Wood consists of thinly bedded limestones, in which 

 the planes of stratification are well marked, hence the process of 

 doloniitization has welded material oiiginally thinly bedded into 

 more massive blocks. In the whole district between Gratton and 

 Cromford it is found generally that wherever dolomite passes laterally 

 into limestone the latter exhibits thinner bedding and more distinct 

 stratification than does the former. 



Streaks and patches of dolomite, which, for convenience, may be 

 termed dolomitic epiphyses, and extensive patchiness on a large 

 scale, features more or less associated with typical vein 

 dolomitization, if present, are obscured. Only at one locality have 

 I found anything approaching clearly marked pseudo-brecciation 

 or patchiness on a small scale. At Winstar occurs a rock con- 

 sistuig of light yellow patches alternating with dark grey areas, 

 both yellow and grey patches usually measuring 1 or 2 inches 

 across. Microscopic and chemical examinations shovv that both 

 light and dark portions consist of minutely granular dolomite. The 

 grains are mainly allotriomorphic and exceedingly small, the average 

 diameter of a grain being about 0'02 mm. The patchiness is not due 

 to any differential ^ dolomitization, but to variable limonitization. 

 The remarkably small size of the dolomitic grains and their 

 allotriomorphism might be considered to indicate that this rock 

 may be a dolomitic mudstone of primary clastic origin, but if such 

 were the case it would be difiicult to explain its lateral non- 

 persistence in the area. Certain megascopic features bearing 

 relation to porosity must be considered here. The dolomitic material 

 of Gratton, Winster, and Matlock (also that in the Harborough 

 area described later) weathers with a characteristic vesicular 

 structure. These cavites vary from a fraction of an inch to several 

 inches across, and appear to be surface phenomena only, since 

 recently worked material in quarries at Winster and near Elton 

 shows none of the vesicular structures. It seems a legitimate con- 

 clusion that the vesicular exterior is produced by the removal of 

 free calcite in the usual processes of sub-aerial denudation. This 

 differential weathering appears to depend upon there being 

 sufficient but not too much free calcite. 



Microscopic Characters. 



The bulk of the dolomitic material exhibits moderately idiomorphic 



crystals varying in size between a minimum width of 0"08 mm. and 



a maximum width of 0-7 mm." PI. IV, Fig. 1, from a specimen taken 



from the Winster escarpment, illustrates the typical structure 



^ In this paper the term " differential " applies to dolomitization which 

 has differentiated between certain portions of rock because of variations in 

 texture or mineraJ constitution. The term " selective " is restricted to cases 

 in which the differentiation is between matrix on the one hand and organic 

 structures on the other. 



