165 



is sometimes a useful test in discriminating between this mineral and pyroxene. 

 The olivine of the gabbros is usually in the form of grains and granular 

 aggregates ; that of the dolerites and basalts may occur as grains, granular 

 aggregates, or crystals. In the plutonic rocks the mineral is frequently 

 traversed by cracks, along which magnetite has separated out and sometimes 

 minute opaque stellate and dendritic inclusions (see ante p. 30) occur in the 

 substance of the crystal. Professor JUUD has called attention to the fact that 

 the olivines of the gabbros are frequently rendered opaque by this separation of 

 secondary magnetite. In the consolidation of the basic magma olivine is always 

 one of the first minerals to form. It precedes in date the pyroxenes and the 

 felspars, and often occurs as inclusions in these minerals. The alteration of 

 olivine to serpentine has already been described in treating of the peridotites 

 (see ante p. 85). The occurrence of certain minerals (tremolite, actiriolite, 

 anthophyllite and pyroxene), in the form of zones round the olivines of certain 

 gabbros and allied rocks has been described by TORXEBOHM, (I) BECKE, (2) 

 ADAMS/ 31 and WILLIAMS.'^ It is regarded by ROSEXKUSOH as a result of pressure 

 metamorphism, and by WILLIAMS as an original structure, dating from the 

 time of the consolidation of the magma. Professor BOXXEY has remarked on 

 the general absence of olivine from the hornblendic gabbros of the Lizard, and 

 suggested that this absence may be due to the fact that olivine as well as 

 diallage may have contributed to the formation of hornblende. 



Hornblende. This mineral occurs as an original and also as a secondary 

 product in basic eruptive rocks. Original hornblende is comparatively rare. 

 It occurs as crystals and crystal fragments or as more or less rounded grains 

 which have lost their crystalline outlines by the action of the magma on 

 previously formed crystals. The forms in the zone of the 

 vertical axis are those of the prism (110), the clinopinacoid (010), and 

 less frequently the orthopinacoid (100). The crystals are usually elongated in 

 the direction of the vertical axis and terminated by clinodomes (Oil), or by a 

 combination of a hemipyramid (111) and the basal plane (001). The prismatic 

 angle for all hornblendes is approximately 124 30'. The original hornblende 

 of the basic rocks is a deep brown colour, similar to that already described in 

 speaking of the picrites (see ante p. 91). The pleochroism and absorption are 

 also similar. The prismatic cleavages are always well developed and furnish, 

 therefore, a most important diagnostic character in the absence of definite 

 form. In cross sections of the prism the angles made by the two sets of 

 cleavage cracks are 124 and 56". Longitudinal sections show parallel 

 cleavage cracks and give extinctions varying from 0, when the section is 

 parallel to the orthopinacoid (100) to 20 or less, when the section is 

 parallel to the clinopinacoid (010). Sections out of the zone 100 : 001 are 

 distinguished by the fact that the extinction position bisects the angles formed 

 by the cleavage cracks ; and, as the optic axial plane is the plane of symmetry, 



(1) N.J., 1877, p. 383. 



(2) T.M.M., 1882, vol. IV pp. 330, 355, and 450. 



(3) American Naturalist. 1885. p. 1087. 



(4) A.J.S., 188(5, p 35. 



