163 



igneous rocks nothing strikes one so much as the evident instability of the 

 former mineral at low temperatures, as shown by its tendency to pass over into 

 the latter. An important question arises as to what extent the change is 

 dependent on the agencies generally included under the terms contact and 

 regional metamorphism. It would be a mistake to suppose that such agencies 

 are absolutely necessary to effect the change, (1) at the same time we must bear 

 in mind the important fact that those areas where this change is most 

 pronounced are precisely those that have been most affected by contact and 

 regional metamorphism. This subject will be again referred to when the rocks 

 are described. 



Rhombic Pyroxene. The rhombic pyroxenes enstatite, bronzite, 

 and hypersthene, (see ante, page 87) are not known to play a very 

 important part in the British rocks of basic composition. They occur, 

 however, in the gabbros of the West of Scotland, and in certain finer grained 

 rocks of the doleritic type ; as, for example, in certain portions of the Whin 

 Sill and in allied rocks from Ratho, near Edinburgh, and Kilsyth in the 

 Campsie Fells. Professor JUDD calls attention to the existence of hypersthene 

 in a dolerite which forms a subordinate dyke in the great serpentine dyke of 

 Forfarshire. (2) In the rocks of the gabbro-family the rhombic pyroxenes 

 usually occur in grains without definite external form ; in the dolerites they 

 usually show a more perfect form. It is not, however, until we come to the 

 rocks of intermediate composition, the andesites and porphy rites, that we find 

 the crystals perfectly developed. 



The prismatic angle of the rhombic pyroxenes agrees very closely with 

 that of augite. In the unaltered crystals or grains the two principal cleavages 

 are those parallel to the prismatic faces ; so that, in this respect, there is no 

 marked difference between the two groups of the pyroxenes. The characters 

 of most use in discriminating between the two groups have been already 

 referred to and need not therefore be recapitulated. When crystalline form is 

 developed, the crystals are usually elongated in the direction of the vertical 

 axis. The common forms in the prismatic zone are the orthopinacoid (100), the 

 brachypinacoid (010), and the prism (110). The prismatic faces are, as a 

 rule, less developed than the pinacoids. The crystals are sometimes seen to be 

 terminated by two faces (brachydomes) meeting at an obtuse angle. As the 

 crystals are usually placed for reference the vertical axis corresponds with the 

 least axis of elasticity (<"="/), and the mean axis of elasticity with the macro- 

 diagonal (b=fi). The optic axial plane is therefore parallel with the brachy- 

 pinacoid (010). A cross section of the prism of a rhombic pyroxene shows the 

 two sets of cleavage cracks meeting at an angle of about 90, and gives in 

 convergent light a positive bisectrix. In enstatite and bronzite this is the acute 

 bisectrix; in hypersthene it is the obtuse bisectrix. By alteration the rhombic 

 pyroxenes develope a cleavage or lamination parallel to the brachypinacoid. In 

 the bron/ites and hyperstheues of the rocks of the gabbro-family we sometimes 



(1) Thus the change to uralite is found in districts where these agencies have not 

 operated. 



(2) Q.J.G.S., vol. XLL, ISSo. p. 39!). 



