Hardman and Hull — A Hitherto Undescrihed Mineral. 167 



xmder the microscope, is scarcely to be identified without the aid of a 

 high magnifying power. 



Microscopical appearance. — Under the microscope, and with a low 

 power (25 diameters), the slice is seen to consist of light brown 

 augite, without crystalline form, in which are imbedded short prisms 

 of plagioclase, imperfect crystals of olivine, and a very few grains of 

 magnetite. But the most abundant mineral is that now for the first 

 time described by Mr. Hardman. It is of a dark umber brown colour, 

 almost opaque, except at the edges, and it forms not only individual 

 masses, but permeates the whole rock, filling the interstices, and 

 enclosing the other minerals. In one instance, where it has appa- 

 rently filled in a cell in the rock, the central portion is vacant ; but it 

 often forms considerable masses. As it does not polarize, it cannot be 

 considered as in a crystalline molecular condition ; and in its distribu- 

 tion, and relation to the other minerals, it assumes very much the 

 -character of amorphous chlorite. Like chlorite, also, it has every 

 -appearance of being a secondary mineral, formed after the consolida- 

 tion of the rock, and with a high power shows a stalagmitic or 

 chalcedonic structure, with wavy bands. 



One of the most interesting circumstances regarding this rock is 

 the abundance of olivine in its unaltered condition. In no other 

 instance, amongst the basalts and dolerites of Antrim which I have 

 examined, have I found it so abundant, and in its original state 

 ■olivine, as is well known, is a mineral very liable to decomposition, and 

 generally it has been completely removed, the outer form only being 

 preserved. In the case, however, of the basalt of Carnmoney Hill, it is 

 as abundant and as fresh as in the lavas of Yesuvius. This can be de- 

 termined by the aid of the polariscope, by means of which the crystal- 

 line grains of olivine are separated out from the augite with which it 

 might otherwise be confounded ; but under polarized light, not only 

 may the outline of the crystalline forms be recognised, but the 

 mineral affords (on rotating the analyzer) the well-known alternation 

 of colours, from ruby red to sap green, characteristic of this mineral. 

 On the other hand, the colours of the augite are blue, grey, light 

 pink, and yellow. The crystalline forms of the olivine are only im- 

 perfectly developed. The crystals of plagioclase — probably labra- 

 •dorite felspar — are well ancl sharply defined, and seem to have 

 crystallized out before those of augite and olivine. "With the polari- 

 scope they show the usual parallel-banded structure, varying with the 

 angle of the analyzer. 



From the remarkably fresh appearance of the olivine one might 

 infer that this rock was comparatively recent, did we not know, from 

 physical considerations, that it must be older than the Glacial and 

 Pliocene periods. 



