part 3] COMPOSITE sill at newton abbot. 265 



alters to a pale hornblende giving extinctions of 15°, and low 

 yellows of the first order : this is referable to tremolite ; accom- 

 panying this is a similar hornblende giving straight extinctions 

 and more fibrous with rathei' less colour, probably anthophylhte. 

 Another colourless augite occurs as subhedral crystals with straight 

 extinction, accompanying the larger olivines ; it is positive and 

 without pleochroism, exhibiting the characters of an enstatite. 



The biotite occurs in strongly coloured red-brown laths, a few 

 of which are partly bleached to a pale crimson-brown; it is biaxial 

 with a small angle, and pleochroic in dark chestnut-brown. 



The felspar varies in amount, some of the rock approaching the 

 peridotites ; it gives extinction-angles in the fcAv symmetrical 

 sections observed up to 30^, indicating a basic labradorite. Much 

 of it is strongly zoned, and some of the felspar ground is probably 

 nearer anorthite. Apatite occurs in short blunt prisms. The 

 iron-ore is always opaque, and the presence of chromite or any 

 other spinel has not been established. Serpentine after olivine 

 forms about 10 per cent, of the rock. 



A Rosiwal analysis of a section was undertaken, and is compared 

 with an analysis by Busz : — 



I. II. 



SiOa 



40*12 



40-24 



AU)^ 



776 



9-59 



FeO 



MgO 



CaO 



NasO 



K2O 



16-01 



23-69 



6-53 



1-20 



0-53 



17-94 



22-04 



7-20 



1-23 



0-50 



HoO 



4-03 





TiOa 



0-37 







P.O. 



0-18 



, 











100-42 



98-74 



Specific gravities = 2'96 and 2-97. 

 I. Picrite, Knowles Hill, Newton Abbot. Busz. (Partial analysis.) 1 

 II. Do. do. do. do. do. Rosiwal analj^sis. 



(b) Xenoliths in the dolerite. — Slides from the outer parts 

 of the xenoliths show nearly the normal character of the dolerite. 

 The augite is subophitic ; occasionally it may be subhedral, and is 

 not quite fresh. Some small tracts of chlorite or serpentine are 

 referable to olivine ; the felspar is andesine, giving extinction -angles 

 up to 20°. 



Slides from the central parts show a much greater proportion of 

 ferromagnesian minerals. The augite is in part poikilitic to the 

 smaller olivine-pseudomorphs ; in general it is subophitic. In 

 PI. XI, fig. 1 is seen a large pseudomorph after olivine, mainly 

 serpentine with some residual magnetite. A hornblende with very 

 pale-green pleochroism is secondary after either augite or the 



^ Quoted by J. P. Iddings, ' Igneous Rocks ' vol. ii (1913) p. 337. 



