16 Notes on the Diabase Rocks 



separations were bordered by a bright green fibrous mineral, 

 which extended irregularly into the colourless portions. 

 This fibrous mineral was strongly dichroic : dark metallic 

 green when the fibres were parallel to the polarising plane, 

 and light green when they were perpendicular to it. This 

 alteration I regard as some chloritic mineral. In further 

 alterations the form of the prism remains; but when ex- 

 amined by polarised light it reacts faintly as an aggregate, 

 and the flaws and separations are lined with brown iron 

 ore. 



The only inclusions which I have observed were small 

 crystals and prisms of magnetite ; but as these were in- 

 variably in the altered parts, and in connection with the 

 chloritic mineral, I regard them as secondary products. 



I think there cannot be any doubt that this orthorhombic 

 mineral is enstatite, and that its alterations are, in the first 

 place, to bastite, and finally to chlorite and to ores of iron. 



Finally, the only remaining mineral to be noticed is 

 apatite. Little, however, need be said concerning it. It 

 occurs in its common prismatic form or as small needles. 

 It is to be observed in the ground-mass, and especially in the 

 larger plagioclase crystals. I have but rarely observed it in 

 the pyroxenic minerals. It varies in amount in different 

 samples of rock — being present in some cases in unusual 

 abundance. The two analyses accompanying this paper 

 illustrate this statement. 



The secondary minerals to be noted are — First, calcite, 

 which either forms pseudomorphs, generally after augite ; 

 less frequently replacing parts of the triclinic felspars ; often 

 filling space or diffused throughout the mass of the rock. 

 It generally forms minute granular, but also crystalline, 

 aggregates, in which the faint chromatic banded effects due 

 to twinning, according to — J^R, are often visible. Next to 

 calcite are the alteration products after enstatite, of which 

 I have spoken. Viridite, to apply a convenient term for 

 otherwise undetermined chloritic minerals, is not so 

 frequent in occurrence as might have been expected in rocks 

 of the Diabase group of such great geological age. It occurs, 

 however, more plentifully in some cases than in others ; and 

 I found it most commonly in slices, taken from places where 

 the limestones rested upon the igneous rocks. Perhaps one of 

 the secondary minerals which is most frequently met with 

 in these rocks is agate, forming amygdules of from over 

 an inch in diameter down to microscopic size. In thin 



