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rounded angles. Here and there it is seen to be partially changed to 

 leucoxene. Apatite frequently occurs in good sized prisms. The 

 texture of the rock is on the whole granitic ; at the same time we 

 frequently see a tendency on the part of the felspar to assume crystalline 

 faces. Lath-shaped sections are common in certain slides, and when they 

 occur they frequently penetrate the diallage ; thus giving the rock a 

 sub-ophitic texture. 



We have now to consider those massive igneous rocks which resemble 

 gabbro in composition, but differ in texture and mode of occurrence. These 

 rocks vary from coarsely crystalline to compact, and from holocrystalline 

 to vitreous. They occur as intrusive lenticles, sheets, dykes, veins, necks, 

 and lava flows. They constitute the great dolerite group of Mr. ALLPOET. 

 No hard and fast line can be drawn between them and the gabbros ; and 

 cases not unfrequently arise in which it is impossible to decide whether a 

 rock should be termed dolerite or gabbro. A typical dolerite is a distinctly 

 crystalline rock, composed essentially of plagioclase, augite, and magnetite 

 or ilmenite. The plagioclase occurs in forms which give lath-shaped sections. 

 The augite may be present as grains, irregular masses or crystals. A typical 

 gabbro, as we have already seen, is a coarsely crystalline rock of similar 

 composition, but of granitic texture. Both the felspar and the pyroxene 

 occur as grains of tolerably uniform dimensions in the different directions. 

 Moreover, in gabbro the pyroxene is usually diallage. It is important to 

 note that these broad differences in texture and mineralogical composition 

 are usually associated with differences in mode of occurrence. Gabbro is 

 found .in large amorphous masses, and dolerite in intrusive sheets, dykes 

 and massive lava-flows. 



If now we fix our attention on the rocks of the dolerite group we note 

 that, in a broad and general way, there are differences between the rocks 

 which occur as intrusive sheets, lenticles, and dykes, and those which occur 

 as lava-flows. The former are distinctly, and in some cases even coarsely 

 crystalline ; they contain little or no amorphous interstitial matter ; and 

 in the majority of cases are not porphyritic. The latter are compact ; they 

 frequently contain a considerable amount of interstitial matter, and are 

 often porphyritic. In some cases basic lavas assume a coarsely crystalline 

 aspect in consequence of the extraordinary abundance of the porphyritic 

 constituents. Careful examination, however, will show the presence in all 

 these cases of a certain amount of fine-grained ground-mass between the 

 porphyritic crystals. It must not be forgotten that the above statements 

 with regard to the differences between intrusive and interbedded rocks 

 are only true in a broad and general sense. Numerous exceptions will be 

 pointed out in the detailed descriptions of the rocks. 



The dolerite group is extremely rich in varieties. These varieties may 

 be due to differences in original characters, or to the more or less complete 

 obliteration of the original characters by the various metamorphic agencies. 

 Consider first the differences between varieties of unaltered or comparatively 

 little altered rocks. The most basic dolerites contain about 45, and the 

 most acid about 55 per cent, of silica. The former have a specific gravity 



