260 



should be referred. A micro-pegmatitic intergrowth of quartz and felspar is 

 often recognizable in the diorites. It is generally assumed that the felspar 

 so intergrown is orthoclase ; but this is certainly not always the case. 



In the altered rocks the felspar is generally more or less turbid and 

 very frequently it has been so changed as to have lost its distinctive 

 optical character. It is in many cases impossible to determine the nature 

 of the extremely minute granular or scaly substances which produce the 

 cloudiness ; in some cases, however, a white mica may be seen to arise in 

 consequence of the alteration of plagioclase. When, as frequently 

 happens, the alteration of the felspar is accompanied by that of the other 

 constituents of the rock chlorite, epidote, calcite and limonite may be 

 produced. 



Quartz. This mineral is sparingly present in the diorites, at least in 

 the form of an original constituent. It occurs as irregular grains filling up 

 the interstices between the other constituents and also as a constituent of 

 micro-pegmatite. Silica also occurs in the form of tridymite, opal, hyalite, 

 chalcedony or jasper in the andesites. Tridymite occurs in the form of 

 overlapping hexagonal tablets in the more or less open spaces. Opal and 

 chalcedony in veins and also disseminated through the mass of the more 

 porous rocks. Professor VON LASAULX and Dr. HATCH have shown that 

 the high percentage of silica in certain andesites is due to the impregnation 

 of the original rocks with secondary silica. In deciding on the position of 

 a rock from the bulk analysis it becomes of the greatest importance to 

 determine whether any silica has been introduced. 



Hornblende. In the andesites this mineral occurs as one of the 

 porphyritic constituents. The usual forms in the vertical zone are those of 

 the prism (110) and clinopinacoid (010) ; less frequently we find these in 

 combination with the orthopinacoid (100). Twinning is not uncommon. 

 The colour in ordinary light may be brown, yellow, orange or green. One 

 very common and interesting feature is the presence of a zone of magnetite 

 granules round the external boundaries. This zone is due to the action of 

 the magma on the crystals. In some cases the hornblende crystals are 

 represented by pseudomorphs of magnetite which retain the form of the 

 original mineral ; in others they are represented by dark patches in which 

 only obscure traces of the form of the hornblende can be detected. Good 

 illustrations of these phenomena may be observed in the hornblende- 

 andesite (porphyrite) of Ben Nevis. YON LASAULX^ OEBEKE< 2 >and others 

 have observed microlites of secondary augite associated with the granular 

 magnetite in the zones surrounding certain hornblendes. 



That the magnetite zone, with or without augite microlites, is the direct 

 result of the action of the magma on the enclosed crystals has been 

 demonstrated experimentally by DOELTER and HUSSAK^) who have produced 

 it artificially. It is quite clear therefore that the hornblende crystals in 



(1) Der Etna. Leipzig (1881), II., 484 



(2) N. J. (1881.) Beilage Band I., 474. 

 (3). N. J. 1884, Band I., 25. 



