58 



designates by the term ophitic. It is well seen in the " ophites " of the 

 Pyrennces. In this country we find it admirably developed in the dolerites 

 (diabases) of all ages from the Cambrian, or possibly even pre-Cambrian, down 

 to the Tertiary period. Thus, it is well exhibited in the diabases 

 (altered dolerites) associated with the Cambrian and Silurian rocks of Wales, 

 in similar rocks associated with Carboniferous strata of Derbyshire and in 

 the Tertiary dolerites of the west of Scotland. It is equally characteristic 

 of the dolerites of the Faroe Islands and of Iceland. (2) 



In all these cases the texture is due to the mutual relations of the two 

 minerals, felspar and pyroxene. The pyroxene, as observed in thin sections, 

 occurs in large irregular plates, and the felspars in lath-shaped forms (see 

 Plate X., Fig. I.) The felspars penetrate the plates of pyroxene in all 

 directions and are frequently enclosed within the pyroxene- substance. Each 

 large plate (ophitic plate as it may be called) of pyroxene possesses uniform 

 optic orientation and therefore extinguishes in one position under crossed 

 nicols. The crystals of felspar on the other hand possess independent 

 optical characters. It is obvious that this texture implies that the felspar 

 has crystallised before the pyroxene. The former mineral shows more or 

 less definite crystalline form it is automorphic the latter mineral plays, 

 as it were, the role of ground-mass, and in crystallising has taken its form 

 entirely from the disposition of adjacent crystals it is xenomorphic. The 

 texture is especially characteristic of the diabases of Continental petrographers 

 and is designated diabasich-kornig by Professor ROSENBUSCH. 



Although the term ophitic, which will be employed in the present work, 

 was originally applied to this texture so far as it is dependent on the minerals 

 felspar and pyroxene, there seems no reason why it should not be extended 

 to similar associations of other minerals. The essential feature of the texture 

 is th,e occurrence of large irregular crystalline masses of one mineral 

 interrupted by detached crystals or grains of a second mineral. It differs 

 from the pegmatitic texture in that the different portions of the second 

 mineral possess independent optical characters they do not stand related 

 to each other as different portions of one and the same crystal. (1) If this 

 extension be admitted then the following pairs of minerals also give rise 

 to the ophitic texture : olivine and bronzite, olivine and hornblende, olivine 

 and augite, olivine and felspar. 



The term granulitic, as applied to texture has been used differently 

 by different authors.. In certain granitic rocks, especially those which 

 contain white mica as one of the constituents, the ground-mass, as it were, 

 of the rock is frequently composed of a number of crystalline grains of 

 uniform size. The appearance under the microscope of this ground-mass 

 reminds one of that of many quartzites. Under polarised light it appears 

 as a brilliantly coloured mosaic if the slide be moderately thick. An igneous 

 rock wholly or partially composed of crystalline grains of this character is 

 said by M. LVY to possess a granulitic texture. Professor JUDD uses the 



(1) BKEON. Geologic d' Iceland, Paris, 1885. 



(2) There also "appears to be a genetic difference between the two textures ; thus the 

 peginatitic texture appears to be due to simultaneous, the ophitic to successive crystallisation. 



