324 DE. F. DIXEY 0>"" THE [vol. Ixxviil^ 



The felsjmr of the ordinarv dole]'ite is of two kinds : the one 

 occurs as laths and is turbid, whereas the other is interstitial and 

 almost water-clear. The laths have a narrow clear margin, which, 

 is opticallv continuous with parts of the adjacent interstitial 

 felspar. Thus" the felspar-laths are strongly zoned, and where the 

 margin of the microscope-slide cuts across one of them, it is 

 interesting to trace the yariation in refractive index from the 

 centre of the crystal to its outer limit. In this way it can be- 

 demonstrated, just as in the case of certain Scottish quartz- 

 dolerites,^ that the composition of the laths ranges fi'om basic 

 labradorite to acid oligoclase. The felsj^ars making up the larger- 

 areas of clear interstitial material range from oligoclase to ortho- 

 clase in composition, and they are riddled by numerous needles of 

 apatite. Some of these felspars, determined as orthoclase, have 

 short rectangular outhnes ; while others possess a curious streaky 

 appearance suggestive of anorthoclase. 



These clear areas, and the minerals that compose them, closely 

 resemble the interstitial jDortious of the AVliin Sill and of certain 

 Scottish and Pembrokeshii'e quartz-dolerites. 



The iron-ores of the dolerite are ^enei-allv abundant; thev 

 occur as microliths, grains, and irregular masses, the last-mentioned 

 sometimes assuming curious Avheel-like and other forms similar to 

 those described by 3Ir. G. W. Tp-rell from the Kilsyth -Croy 

 intrusions.'-^ 



(3) The ^Minerals of the Complex. 



Q ua r t z. — In the aplite- veins, particularly at their fine extremities,, 

 quartz occurs in fair amount, both in granular form and in gi-aphic 

 intergrowth Avith felspar : the felspar of the intergrowth is 

 generally orthoclase. but in some eases it is an acid plagioclase. 

 In the norite. however, quartz does not occur as a normal product 

 of crystalhzation ; this is only to be expected, since olivine is so 

 commonly developed. Xevertheless, it is interesting to note that 

 there was probably a small excess of silica at the time when the 

 feLspars were crystallizing ; later, as the felspars attained a lower 

 temperatui-e, this excess was precipitated in the form of strings. 

 and small patches of granular quartz. 



Felspars. — The only felspar of the normal norite is an acid 

 labradorite the composition of which is near Ab,An , as determined 

 by extinction-angles and refractive index. This felspar is fresh and 

 clear, grey in colour, and free from zoning. It generally forms 

 pinacoidal tables with Carlsbad and albite twinning ; pericline 

 twinning occurs also. The grey colour of the felspars in the hand- 

 specimen is due to the presence of innumerable minute opaque 



^ ' The Geology of the Xeighbourhood of Edinbnrgh ' 3Iein. Geol. Snrv^ 

 Scot. 1910. p. 30^5. 



- '■ Geologv & Petrology of the Intrusions of the Kilsyth-Croy District 

 Pumbai-tonshire) ' Geol. 5lag. 1909, p. 308. 



