﻿518 DE. P. H. HATCH AND ME. E. H. EASTALL ON [Nov. 1 0, 1 0, 



in the Chalk of Antrim. Mr. Whitaker l also refers to the occur- 

 rence of pebbles of various rocks in the Chalk near Gravesend ; 

 and mention may be made in this connexion of the well-known 

 occurrence of boulders of igneous and other rocks, often of large 

 size, in the Cambridge Greensand. 2 



During the period in which the dolomite and its included blocks 

 were maintained at a high temperature, there must have been 

 a molecular interaction between the latter and the former. The 

 materials derived from the granite block are silica, alumina, and 

 iron. The influence of the iron (derived, no doubt, chiefly from 

 the pyroxenic constituent of the granite) did not extend beyond 

 the innermost or dark-mica zone of the reaction-rim, where, to- 

 gether with silica and alumina, it. is combined with the magnesia 

 of the dolomite to form an iron-bearing phlogopite and olivine. 

 In this zone the alumina has entered into combination also with 

 magnesia alone, to form spinel. It also plays a part in the light- 

 mica zone, in which it forms a non-ferruginous phlogopite ; while 

 forsterite has been formed in this zone by the union of silica 

 with magnesia but without alumina. The influence of the silica 

 alone extended to the outermost zone, where it forms forsterite. 



The decomposition of the double carbonate of the dolomite 

 liberated carbonate of lime, which crystallized as calcite in all three 

 zones. 



To test this inference, some of the white carbonate material was 

 taken from the dark-mica zone, coarsely crushed, the fine dust 

 removed by washing, and the residue dried. From this material 

 the mica and olivine were removed by means of a powerful electro- 

 magnet. The residue was shaken up with bromoform (diluted 

 with benzine to a density of 2-79) 3 : the whole of the material 

 immediately rose to the surface. On the other hand, material 

 from the pure dolomite-rock, treated in a similar manner, separated 

 into two distinct portions, the smaller part floating in the bromo- 

 form, the other sinking to the bottom. Since calcite has a density 

 of 2*72 and dolomite one of 2*85, it is obvious from this experi- 

 ment that the white material of the dark-mica zone is almost 

 entirely calcite. A similar result was arrived at by the use of the 

 Lemberg staining test. It was found that the whole of the car- 

 bonate in a section of the dark-mica zone acquired the stain, while 

 in a section of the pure dolomite-rock, treated simultaneously, the 

 bulk of the carbonate remained uncoloured. 



The nature of some of the chemical reactions which took place 

 between the dolomite and the silica, alumina, and iron of the inclu- 

 sions may be theoretically represented by the following equations : — 



1 • Guide to the Geology of London' Mem. Geol. Surv. 6th ed. (1901) p. 31. 



2 W. J. Sollas & A. J. Jukes-Browne, Quart. Journ. Geol. Soc. vol. xxix 

 (1873) p. 11 ; P. F. Kendall, Final Rep. Roy. Coal Commission, 1905, pt. ix, 

 App. 3. 



3 A heavy medium suggested for the purpose by Mr. T. Crook, of tho 

 Imperial Institute. 



