BY THE REV. J. MILNE CURRAN. 221 



base resolves itself into tiny globules of olivine, augite and felspar. 

 In this ground-mass augite is much more abundant than would be 

 supposed at first sight. In every slice black grains of magnetite 

 are plentiful, and remain opaque in the thinnest sections. It is 

 usually well preserved and shows no signs of decomposition. It 

 was the first mineral to separate from the glassy magma, and is 

 the only primary constituent ever enclosed in the olivines. 

 Broadly speaking, the structure is decidedly micro-porphyritic. 



A " streaming of the felspars " is a very characteristic structure 

 at once recognised under the microscope. It is hardly pronounced 

 enough, however, to be termed a fluxion structure. The lath- 

 shaped plagioclases are often seen sweeping round the larger 

 olivines aud augites, pointing, without doubt, to movements in the 

 molten magma. This structure is shown on PI. xiv. figs. 4 and 5. 



The abundance of black magnetite which remains opaque even 

 in the thinnest slices is the next feature to attract attention. 

 From Professor Judd's researches, I could conclude from this 

 feature alone that the rock cooled at or near the surface. As 

 Professor Judcl remarks,* in most deeply-seated rocks the iron 

 oxides enter into complete combination with the silicates, and in 

 other cases there is a progressive increase in the quantity of 

 magnetite which is separated according to the proximity to the 

 surface at which consolidation has taken place. 



Magnetite was one of the first minerals to separate from the 

 magma. It is the only mineral ever included in the olivine, but 

 its enclosure in this mineral is a very common occurrence. The 

 large olivine crystal on PI. xiv. fig. 4, shows a cube of magnetite. 

 The felspars, too, it will be noticed, seem for the most part per- 

 fectly fresh and unaltered. These plagioclases exhibit parallel 

 twin-striation in polarized light, a feature common to rocks of 

 this sort throughout the globe. 



The olivines are abundant in every slice, showing, as is usual, 

 that peculiar ground glass surface which helps to identify it. 



* Q.J.G.S., Vol. xlii., p. 88. 



