﻿514 DR. F. H. HATCH AND MR. R. H. RASTALL ON [Nov. I9IO, 



which has taken place ; the original dolomite has completely disap- 

 peared, and is replaced by an aggregate of mica and forsterite, the 

 interstices between the crystals being filled with secondary calcite, 

 in plates which are optically continuous over large areas (see 

 p. 518). 



The reason for the difference in colour of the two mica-zones is 

 clearly brought out under the microscope ; the mica in this case 

 is absolutely colourless, and the olivine mineral is serpentinized 

 along cracks without separation of iron-ore, a fact which points 

 strongly to its being the pure magnesian orthosilicate, forsterite. 



Besides the calcite plates, which are evidently of secondary 

 origin, there are a few rounded grains of carbonate enclosed in the 

 forsterite ; possibly these are unaltered remnants of the original 

 dolomite. 



Towards the outer portion of the zone the serpentinization of 

 the forsterite grains increases rapidly, a few grains being more 

 than half replaced by serpentine. 



(d) The ophicalcite zone. — This zone consists of an aggre- 

 gate of white carbonate and greenish-yellow serpentine, which, 

 under the microscope, is, by its form, clearly recognizable as deriva- 

 tive from forsterite. It shows the characteristic mesh-structure in 

 great perfection, and in one grain a core of the unaltered mineral 

 was identified as forsterite by its high refractive index, strong 

 birefringence, and absence of colour. 



The serpentine in ordinary light is quite colourless, and is distin- 

 guishable from the carbonate by its more perfect transparency. 

 Between crossed nicols it shows rather weak birefringence. Figs. 3 

 & 4 in PL XXXV illustrate the appearance of a section in ordinary 

 light and between crossed nicols respectively. 



(3) Other Inclusions in the Dolomite. 



In other parts of the quarry, specimens were obtained showing 

 interesting associations of magnesia and lime silicates, which, as in 

 the instance already described, evidently owe their origin to action 

 between the dolomite and enclosed pieces of foreign rocks. Two 

 specimens merit detailed description : one a felspar - scapolite- 

 diopside-rock, and the other a rock largely composed of forsterite 

 and spinel. 



(a) The felspar-scapolite-diopside rock. — This consists 

 mainly of a pale bluish-grey aggregate of felspar, separated from 

 the dolomite which encloses it by a zone of silvery mica about 

 an inch wide ; in this zone the mineral occurs in long narrow 

 blades disposed radially, that is, at right angles to the boundary of 

 the inclusion. Examined in convergent light, the mica is seen to 

 have so small an optic axial angle as to be practically uniaxial. 

 There is little doubt that it is the magnesian mica, phlogopite. 



Examination of a thin section of the felspar-rock shows that it 

 is, in the central portion, almost entirely composed of a plagioclase 



