268 ME. T\'. G. sHAxyox o^ A [vol. Ixxviii, 



of the slides (X 20) ^ there are two generations, smaller ciystals 

 ^vhich are eiihedral and showing diallage striation, and a later 

 ophitic generation : in N 18 the angite is ophitic throughout. The 

 plagioclase is of an acid yariety, giving extinction-angles in sjm.- 

 metrical sections up to 15°, with a refractive index higher than 

 quartz, and is referable to andesine ; in jST 15 this is acid, in 

 ]Sri8 it becomes more basic. Orthoclase appears to be present in 

 most of the slides — in X 15 it forms about a third of the felspar, 

 showing gradation towards a monzonite facies. 



Accessor}^ minerals are quartz, iron-ores, and apatite. The 

 bigger grains of quartz shoAV corrosion -borders with granular 

 epidote or chlorite, and deep inlets occupied by chlorite ; the 

 smaller grains are anhedral to the felspar, and contain prisms of 

 apatite of the same size as in the normal rock. The amount of the 

 quartz varies, in some cases amounting to at least 3 per cent. 

 The iron-ore may be either ilmenite or pyrites, rarely magnetite. 

 Secondary minerals are epidote in considerable quantit}" after 

 felspar, and chlorite after augite. The texture is generally rather 

 fine-grained, becoming coarser with the occurrence of euhedral 

 augite. Specific gravity=:2'81 and 2'84. 



A further variety of the dolerite occurs as a small outcrop on 

 the flank of the picrite south-east of the field, adjacent to the 

 quarry. The sections cut var}^ considerably, but all contain some 

 olivine. The latter is unevenly distributed ; in X 21 olivine is 

 the chief f erromagnesian mineral, in N 22 augite is in much 

 greater proportion. Enstatite generally" accompanies the olivine 

 when it occurs in considerable quantity. Part of the felspar is 

 much decomposed ; some undoubted andesine occurs, and is much 

 fresher. The decomposed felspar shows no twin striation, and is 

 considered to be orthoclase — in X22 this forms about half the 

 felspar. A fair amount of chlorite is present, and the iron-ore 

 is ilmenite. Although the rock is much weathered, the olivine is 

 notably fresh, and is not as much altered as the olivine in the 

 picrite. Quartz forms the last product of cr^^stallization, and 

 contains ajDatite ; further, the felspar is euhedral to it : hence this 

 quartz is original. 



This variety of the dolerite is very restricted in distribution, 

 occurring onh^ in one part in immediate contact with the picrite. 

 All other sections of dolerite, apart from the xenoliths, contain 

 no olivine. This particular variety is a differentiation-facies of 

 the picrite, in which the proportion of felspar is notably increased. 

 But this explanation is not accepted for the rest of the olivine 

 occun'ence in the xenoliths, in view of the facts just stated. 



(y) Lower Quarry: dolerite. — The rocks here are com- 

 pletely chloritized, the original mineral having been augite. 

 The felspar proves to be andesine, where it is fi'esh enough for a 



^ The niunerals N 20, etc. refer to the slides which I have presented to 

 the Museum of Practical Geology, Jermyn Street, London, S.W. 1. 



