108 L. M. Parsons— 



separated by a narrow belt composed of unaltered limestones of a, 

 lower horizon in Dj. 



The metasomatism reaches its culmination in the terraced mass 

 known as Harborough Eocks. In this region the dolomitization 

 appears to be characteristic of a definite part of the sequence and 

 has been described as sucb in one of the earlier Survey Memoirs.^ 



In a later Survey Memoir the opinion is expressed that the 

 dolomitization is not confined to beds of a particular horizon.^ 



Although the dolomitic material of this district exhibits many 

 inherent features similar to those of the rocks of Winster and 

 Matlock, description in a separate section of the paper is justified 

 mainly by differences in horizon and relations between tectonics and 

 field relations. 



Field Eelations. 



Boundaries up and down the sequence. — West of Ballidon Moor, 

 and, again, in the country east of Brassington, where the apparent 

 base of the dolomitic material is brought to the surface by the 

 general dip, there appears to be a sharp line of division between 

 the limestone below and dolomite above. The dolomitization 

 at any particular part of the area so obviously follows the 

 bedding and dip, and is so consistently modified by faulting, that 

 conclusions formed from these prevalent features alone would be 

 decidedly in favour of either primary precipitation or con- 

 temporaneous alteration. Whatever may be the origin of the 

 metasomatism, there is no doubt that any part of the field showing 

 limestone . below and dolomite above exhibits a definite lowest 

 " bed " of dolomite which may be conveniently termed the " hasal 

 dolomite ". Whether the basal dolomite of one locality is on the same 

 horizon as the basal dolomite of any other locality or occurs at a 

 different horizon, is a question that must be settled by reference to 

 the faunal characters of the lowest dolomite itself and of the lime- 

 stone immediately below. 



At Hopton, Carsington, and south-east of Brassington, the basal 

 dolomite is in the D^ part of the sequence and a similar horizon is 

 found in the lowest dolomite near Many stones Quarry, west of 

 Harborough. 



In the vicinity of the Griffe Walk, further north, however, the 

 basal dolomite appears to be in D^ beds. Consequently there is, 

 here, a change in horizon and a fairly steep horizon gradient. In the 

 dolomitic mass faulted down west of Longcliffe, the basal dolomite 

 appears to be mainly in Dg- 



Horizon changes are generally so gradual that they are 

 imperceptible at any restricted portion of the field. It is clear that 

 the Dj portion of the Harborough dolomite has passed laterally into 



1 North Derbyshire, 1887, p. 24. 



^ The Geology of the Northern Part of the Derbyshire Coalfield and 

 Bordering Tracts, 1913. 



