58 L. M. Parsons — 



composed of some idiomorphic crystals together with a large number 

 of rather allotriomorphic grains. Considerable variation in size is 

 also shown. Since much of the material is rather allotriomorphic, 

 the dimensions given here and in the table refer to the largest 

 diameter of grains or to the larger diagonals of rhombohedra. 



TABLE ILLUSTRATING VARIATION IN SIZE OF GRAIN OF 

 DOLOMITIC MATERIAL BETWEEN GRATTON AND CROMFORD. 



The question arises as to whether size and allotriomorphism in 

 themselves yield reliable evidence of origin. 



Mr. E. E. L. Dixon has found that certain contemporaneous 

 dolomites in the Swansea area are composed generally of much 

 smaller grains or crystals than those forming subsequent dolomitic 

 material occurring in the same district.^ This author gives the 

 maximum size of grains in the laminosa dolomites (contemporaneous) 

 as O^Ol inch in length of side. This approximately corresponds to 

 a maximum diagonal of "375 mm. In the case of the Swansea 

 subsequent dolomites the average length of side of their rhombohedra 

 is given as 0'02 in., corresponding to a larger diagonal of "7 mm. 

 If these dimensions are compared with those given in the table 

 above, it will be seen that the Winster and Matlock material is 

 commonly composed of grains smaller than the maximum of the 

 Swansea contemporaneous dolomite, and considerably smaller 

 than the average size of grains in the Swansea subsequent dolomites. 

 If we accept the field evidences indicating that the Winster material 

 is of subsequent origin, it is clear that subsequent dolomites in one 

 area may be composed of grains as small as those composing con- 

 temporaneous dolomites in another area.^ 



The nature of the original limestone is a factor of considerable 

 importance in the questions of relative idiomorphism and size. 

 The transition limestone at Winster (see ?1. IV, Fig. 2) shows 

 that the original limestone there probably consisted of a mass of 

 minute organisms, sponge spicules, foraminifera, etc., embedded 

 in a fine-grained matrix almost completely non-recrystailized. The 

 metasomatism at Winster has evidently led to a considerable 

 increase in size of grain, and in this respect bears out observations 

 of Van Tuyl. 



1 Mem. Geol. Survey : "The Geology of the South Wales Coalfield: The 

 country around Swansea," 1907, p. 16. 



^ See also researches of Van Tuyl, Iowa Geol. Surveij, vol. xxv, p. 395. 



