part 3] COMPOSITE sill at newton^ abbot. 267 



being used : this gave a reading of 72 per cent, for the felspar, 

 agreeing fairly closely with the Kosiwal analysis. 



{d) Fine-grained dolerite (' mugearite ' variation). — 

 In thin section the plagioclase-laths give a trachytic aspect to the 

 section, and the extinction is sensibly straight ; the average length 

 of these laths is about 1*5 mm. The plagioclase is not all of one 

 kind ; in addition to these, more tabular felspars and some inter- 

 stitial grains of unstriated felspar referable to orthoclase occur. 

 In one case this could be established by a Becke test, giving a 

 refractive index lower than that of balsam. The tabular crystals 

 give extinction-angles up to 12"^ or 14°, and are evidently oligoclase- 

 andesine, the refractive index being equal to that of quartz. The 

 lath-felspars form about 35 per cent, of the rock ; their refractive 

 index is below that of quartz, and with the straight extinction this 

 indicates oligoclase. In addition to these, a few rhombs are seen 

 presenting the streaky appearance of soda-orthoclase. A fair 

 amount of chlorite occurs, but no fresh augite ; in some cases the 

 chlorite may be after small olivine. The final product of crystal- 

 lization is quartz moulding some of the felspar, and containing 

 small elongated needles of apatite : hence this is primary. Larger 

 quartz-grains occur, notably corroded, with inlets of the ground 

 and with a border of granular chlorite ; this is xenolithic quartz. 

 Calcite is present in grains. The iron-ore appears to be always 

 magnetite. 



The rock presents considerable affinities to the mugearites, jfas 

 defined by Dr. A. Harker. A partial Rosiwal analysis is compared 

 with the percentages given by him ^ : — 



I. Per cent. II. Per cent. 



Oligoclase 57*5 



Orthoclase 12'5 



Olivine, etc 26'5 



Apatite 3'5 ' 



100-00 



Oligoclase 



Oligoclase- andesine. 

 Orthoclase 



37-53 



11-04 



5-71 



Quartz 



Chlorite 



Magnetite 



Totals 



0-81 



35-51 



9-38 



99-98 



Specific gravitj' = 2-76. Specific gravity = 2-79. 



I, ' Mugearite ' variation of dolerite, Knowles Hill. Rosiwal analj'sis. 

 II. Mugearite, Druim na Criche (Skye). 



The lower specific gravity is probably due to secondary altera- 

 tion. The Knowles-Hill rock is more basic, and analcite was not 

 found, although carefully searched for ; the texture is also coarser. 



(e) Dolerite. — The sections of dolerite display no especial 

 features beyond the occurrence of quartz, both original and xeno- 

 lithic. They are not ver3^ fresh, even at some distance from the 

 surface. The augite is usually ophitic to subophitic, but in some 



1 ' The Tertiary Igneous Eocks of Skye ' Mem. Geol. Surv. 1904, p. 265. 



