160 



The variety of augite known as diallago is especially characteristic of the 

 gabbros. Its distinguishing feature is the presence of a lamination parallel 

 to the orthopinacoid. This lamination is frequently accompanied by the 

 presence of minute brownish plates, which lie parallel with the same plane 

 and give a bronzy lustre to the cleavage surfaces. In the variety known as 

 pseudo-hypersthene similar inclusions occur along more than one set of 

 parallel planes (see ante, p. 29.) 



When affected by surface agencies the augite of basic eruptive rocks often 

 passes into ill-defined fibrous or granular substances of a greyish colour. 

 Another common alteration product is chlorite. This occurs in the form of scaly 

 or fibrous aggregates, which give under crossed nicols a peculiar deep blue 

 shimmering light. It is very common amongst the older plagioclase-augite 

 rocks, and is, indeed, the only mineralogical feature available for 

 distinguishing these rocks from the tertiary dolerites ; and even this feature 

 is not constant. Calcite and minute epidote granules are often found 

 associated with chlorite. The non-aluminous augites may give rise to 

 serpentine. 



The effects of the intrusion of granite upon the augite of basic igneous 

 rocks has been described by Mr. ALLPORT, (I) and his results have been shown 

 to possess a general significance by the work of LOSSEN (2) in the Hartz and 

 Michel-Levy (3) in the Maconnais. The augite passes into uralitic and 

 actinolitic hornblende. The term uralitic is applied when a crystal or grain 

 of augite is converted into an aggregate of hornblende-fibres without the 

 destruction of the original form of the crystal or grain ; the term actinolitic, 

 on the other hand, implies that the original form has been more or less 

 destroyed, and the fibres individualized as distinct needles of actinolite. 

 Speaking of the hornblende developed by contact metamorphism, Mr. ALLPORT 

 says : " Like all hornblende it exhibits colours in polarized light and is 

 dichroic : but the crystals differ in character from those occurring in diorites 

 and other igneous rocks ; nor are they the same as those forming the true 

 hornblende- schist of the Lizard District for example ; they have often the 

 character of actinolite, and are frequently aggregated in radiating groups 

 composed of flat blades of a bluish -green colour, and not very translucent ; 

 occasionally, however, there are crystals quite similar to those in ordinary 

 hornblende-schist." That the conditions under which the metamorphism of 

 the rock was affected admitted of a considerable amount of movement 

 amongst the chemical constituents without, at the same time, producing 

 anything like fusion, is proved by the fact that it is often traversed by cracks, 

 now filled with actinolite, quartz and probably in some cases a colourless 

 secondary felspar. The last sentence quoted from Mr. ALLPORT'S paper 

 is important because it shows that compact hornblende (4) may arise in 



(1) On the metamorphic rocks surrounding the Land's End mass of granite. Q.J.G.S.. 

 vol. XXXII., 1876, p. 407. 



(2) Jahr. d. k. preuss. geol. Landesanstalt fiir 1883 und 1884. 



(3) B S.G., Se. III., Tome XI., p. 273. 



(4) By "compact hornblende" is meant hornblende which is homogeneous; that in, 

 neither fibrous nor actinolitic. 



