76 PEOF. J. W. JUDD 01^ TEETIARY 



and similar conditions can never exist, even in the largest mass 

 extruded at the surface ; thus it comes to pass that we never find 

 lavas assuming the form of gabbro or granite. 



The ophitic type is clearly found in masses which are of con- 

 siderable dimensions; though not so large as those forming the 

 gabbros, in which the rate of cooling is somewhat more rapid and 

 the pressure less than in the case of the latter. But the geological 

 relations of these rocks also indicate that the ophitic structure is 

 produced where the molten mass is in a state of perfect internal 

 equilibrium ; that is to say, where there is no kind of movement 

 whereby strains and tension .can be produced in the viscous mass 

 during its consolidation. Nowhere can such conditions exist in such 

 perfection as in the great masses of lava, sometimes hundreds of 

 feet in thickness, and extending over many square miles in area, 

 which are found intruded between strata. When the injection has 

 once taken place, such a mass must remain in a state of almost 

 perfect internal immobility. In some of the large lava-currents, 

 when a solid crust has formed over them and no further movement 

 can take place, the conditions seem to be almost equally favourable 

 for the formation of this structure, and accordingly we find an ophitic 

 structure, though of a somewhat less perfect character, arising under 

 such conditions (see Plate Y. figs. 3, 5). 



In masses of molten rock where internal movements accompany 

 the crystallizing process, a ""granulation" of the pyroxene and 

 olivine is the result. Instead of the formation of large crystals, we 

 have an agglomeration of grains with more or less marked curvilinear 

 outlines, presenting every possible orientation. That this action 

 really takes place can be shown in gabbros and ophitic dolerites in 

 which movement has taken place towards the end of the process of 

 solidifying (see Plate YII. fig. 4). In these cases the pyroxene 

 crystals are found surrounded by a zone of granulated pyroxene. It 

 is when this crystallization goes on in a mass not in a state of 

 internal equilibrium, that the granulated structure is developed ; and 

 the passage from the granulitic and micro-granulitic dolerites, which 

 are holocrystalline, through the various types of basalt, in which 

 more and more of the vitreous base appears, is as perfect as in the 

 parallel case of the ophitic rocks (see Plate Y. figs. 2, 4, 6). 



Of course, as might be expected, we find every gradation between 

 these two parallel series of rocks, ophitic rocks passing insensibly 

 into granulitic ones. 



The mode of separation of the magnetite, felspar, and augite from 

 the glassy base of the basalt appears to be equally determined by 

 the existence of equilibrium or otherwise in the midst of the cooling 

 mass. In dykes especially, and in some lava -streams, the entire 

 absence of any internal movements in the cooling mass appears 

 to be conducive to the formation of delicately branched rods or 

 skeleton-crystals of magnetite, with the long transparent rods bifur- 

 cating at both ends, which seem to be skeleton-crystals of augite, 

 felspar, or olivine (see Plate YI. figs. 1, 3, 5, 7). Put in the majority 

 of cases, we find microlites of rounded forms of augite and magnetite 



