169 



M. THOULET has given tables (1 > which show the relation between the angle 

 made by the two sets of cleavage cracks and the direction in which the 

 section is cut ; for sections out of the three principal zones, and these tables 

 are often of considerable use in determining the orientation of any particular 

 section.' 2 * 



The colour of the monocliuic pyroxenes varies considerably in different 

 individuals, and sometimes in different portions of the same individual. Some 

 varieties are colourless in thin section, others are brown, yellowish-brown or 

 brownish- violet, and others a pale green. KXOP (3j has recently shown that 

 certain reddish-brown or violet-brown augites contain several per cent, 

 of titanic acid. The augites of the basic eruptive rocks are not as a rule 

 markedly pleochroic. Some of the deeply coloured varieties do, however, 

 show a perceptible pleochroism of the kind referred to in describing the 

 Inchcolm picrite. The hour-glass structure already mentioned as occurring 

 in the picrites is occasionally found in the normal plagioclase-augite rocks. 

 Zonal structure due to a variation in the colour of successive layers is 

 occasionally seen in the porphyritic augites of certain dolerites and basalts. 



Augites showing good external form are comparatively rare in British 

 rocks of basic composition. They occur, however, as porphyritic elements in 

 'lolerites of the Lion's Haunch (Edinburgh) type, and as constituents of the 

 main mass of the rock in those of the Rowley Rag type. The porphyritic 

 augites frequently contain portions of the groundmass, magnetite and 

 sometimes olivine as inclusions. As a general rule the augite of the coarsely 

 crystalline (gabbros) and medium grained rocks (dolerites) occur as large 

 grains or irregular masses (ophitic), and those of the finer grained rocks 

 (basalts) as granules and granular aggregates. We sometimes find in the 

 basalts minute well-formed augite crystals lying in a groundmass of felspar. 

 (see fig. 2, plate XXIV.) This is a common feature in the Tertiary felspar- 

 basalts of the continent, but it is rare or absent from the corresponding rocks 

 in this country. With us, on the other hand, it is found amongst the 

 carboniferous basalts of the Midland Valley of Scotland and the 

 neighbourhood of Limerick, in Ireland. Microlites of augite are sometimes 

 present in rocks with a considerable amount of glassy base. If we compare 

 the rocks of the tertiary with those of the palaeozoic periods in the British 

 Isles, it is interesting to notice that the augites with well marked crystalline 

 form appear to be much more abundant in the latter. It is amongst the 

 carboniferous rocks of the Midland Valley of Scotland and the neighbourhood 

 of Limerick, in Ireland, that we find the most perfect crystals of augite. 

 Ophitic augites are equally abundant in the palaeozoic and tertiary rocks. 



(1) See FouauE and LEVY. Mineralogie Micrographique ; also HUSSAK Einleiteng 

 zum bestimmen der gesteinbildenden Miueialien. p. C4. 



(2) Thus", taking the zone 100 : 001, the angle varies from to 87. By observing the 

 angle and referring to the table, it is possible to determine the inclination of the section to the 

 vertical axis of the crystal. If such a section should be cut at right angles to an optic axis, then 

 the angle which this axis makes with the vertical axis is known, and by combining this observa- 

 tion with the observation of the maximum extinction in the zone of the vertical axis it is possible 

 to obtain an approximation to the optic axial angle. 



(3) Z.K. 1885, p. 58. 



