348 Morris Morris: 



Dolerite Dykes, but they fall into two pairs owing to a marked 

 difference in structure, viz.. ophitic and non-ophitic. 



The Non-Ophitic Dykes comprise a dyke lying just outside the 

 south-west corner of the area mapped, viz., in Allot. 48, and the 

 dyke shown off Cambridge-road in Allot 23^1. The former has a 

 width of at least 4 to 6 feet, and was traced for 50 yards, the 

 latter is not less than 10 feet wide, and was traced for about 100 

 yards. 



These are dark, tine-grained basaltic rocks, with no minerals 

 discernible in hand specimens. Augite occurs in grains through 

 the ground mass, and in larger crystals, with frequent twinning, 

 olivine is now represented by its pseudomorphs. Iron oxide is very 

 abundant, with shape and lustre of magnetite, but the prevalence 

 of leucoxene-like alteration product suggests that the rock is. 

 titaniferous, and that some ilmenite is present. Laths of plagio- 

 clase (chiefly labradorite) are common, apatite occurs sparingly. 

 Ground mass consists of granular matter set in eryptucrystaline tu' 



The Ophitic Dykes comprise a dyke in Allot. 40b, and a dyke 

 lying outside the map, where it penetrates a small residual of the 

 series (a). 



The former is a dark compact rock, showing large phenociysts 

 of augite, and long thin laths of felspar. This augite is greeti, 

 generally zoned, and showing composite structure, often moulded 

 on idiomorphic olivine, and penetrated by laths of basic plagio- 

 cluse. Augite also in long purple, slightly pleochroic crystals, sug- 

 gesting a titaniferous variety. Iron oxide is very abundant. 



The latter is also a dark compact rock, but with much smallci' 

 crystals of augite and olivine. Its structure answeis the descrip- 

 tion of the previous lock as far as the relations of augite, olivine 

 and felspar laths t(j one another are concerned. But there is alsa 

 an al)undance of another mineral occupying interstices, mouded on 

 tlie augite and olivine, and often enclosing the plagiodase laths. 

 It has two cleavages, one perfect, is biaxial and negative, with low 

 pol. colours, and a ref. index between 1.494 and 1.510. and does 

 not gelatinise with H.Cl. tliougli if is decomposed by it. It thus 

 has the iiualities of Stibiti'. .Mtiiough it has the appearance of a 

 priiuiiry mineral, especially as the rock sliows no other signs of 

 allei atioii, it eertaiiily owes its oiigin to the pneuniatolytie action 

 of steam upon a giound mass whose remains are now only spar- 

 ingly developed, and which was elose to plagiodase fel})ar in its 

 constituents and proportions. 



