lf> 7 



II 11. Diallagc i'roiu gabbro. Caerleon Cove, tlio Lizard, Corn\vall. -Mr. 



HUDDLESTON. Q.J.G.S., Vol. XXXIIL, p. 927. 



IX. Augite from the Whin Sill, (Enstatite-dolerite). TEALL, Q.J.G.S., 



Vol. XL., 1884, p. 648. 



It will be observed that most of the British rock-forming augites, of 

 which analyses have been made, come from the Tertiary gabbros of the West 

 of Scotland, and that they belong to the division which is poor in alumina. 

 The augite of the Whin Sill corresponds very closely to the following 

 formula 



14 Ca Fe Si 2 e 1 



5 Ca Mg Si a 6 j 



6 Mg Mg Si 2 6 J. 

 3 Mg AL Si O e 



1 Mg Fe. 2 Si 6 J 



This mineral is remarkable for the large amount of iron, existing 

 presumably in a silicate of the Hedenbergite type, and for the excess of 

 magnesia over that required to combine with the remaining lime and the 

 sesquioxide bases. The excess implies the existence of the silicate Mg Si0 3 , 

 written in the formula as MgMgSi 3 for the sake of symmetry. 

 Whatever theory we adopt as to the constitution of the pyroxenes, we seem 

 driven to the conclusion that this silicate, known to us as the rhombic mineral 

 enstatite, exists in a monoclinic augite. One point is certain, viz. that the 

 substance analysed did not consist of a visible mixture of rhombic and 

 monoclinic pyroxenes. 



Augite occurs in the basic eruptive rocks as crystals, irregular crystalline 

 masses, granules, and granular aggregates. When it occurs as crystals the 

 common forms are the orthopinacoid (100), the prism (110), the clinopinacoid 

 (010) and the positive hemipyramid (111). 



Fig. 1. Augite crystal. - a = (ICO), b = (010), m = (110), s= (ifl). 

 Fig. 2. A twinned crystal of augite. 



The larger crystals are usually short in proportion to their width ; the 

 smaller crystals are often elongated in the direction of the vertical axis. The 

 prismatic angle is the most important for petrographical purposes. It is 

 about 87, and does not vary to any considerable extent in the different 

 varieties, 



