Birmingham Natural History Society. ,433 



long, acicular, hexagonal crystals. The olivine occurs in crystals, 

 and also in irregular grains, of a clear yellowish green colour ; it is, 

 however, rare in this state, being nearly always altered to a dull dark 

 green mineral. In addition to the above minerals, which must be 

 regarded as the original constituents of the rock, there are also one 

 or two zeolites, caloite, and a chloritic mineral, all of which are 

 found filling cavities, and are secondary formations. 



The use of the microscope is by no means confined to the dis- 

 crimination of minerals ; with its assistance we may learn many 

 facts, as to the mode of formation of rocks, the order in which the 

 various minerals crystallized, and the alterations which have been 

 frequently caused by the removal of mineral matter, and its re- 

 placement by another of different chemical composition. In the 

 Eowley rocks, the minute crystals of apatite penetrate both the felspar 

 and augite ; the latter also encloses crystals of felspar and magnetite ; 

 the augite crystallized therefore, after the others had been formed. 

 The olivine contains grains of magnetite only, and was probably the 

 second to crystallize. 



Cases are not uncommon in which crystals have caught up portions 

 of the surrounding mass while in the act of formation, and other 

 facts indicate very clearly the actual condition of the mass at the 

 moment of crystallization. For example, in a section of Pitchstone 

 from Planitz, containing crystals of felspar, the minute opaque 

 particles thickly scattered through the matrix are crowded together 

 round the sides of the crystals, having been forced outwards as the 

 latter increased in size ; this clearly indicates that during the for- 

 mation of the crystals, the matrix was in a viscid, but not in a fluid 

 state, for had the particles been quite free to move, there would have 

 been no crowding. 



In a section of basaJt from the Ehine, the olivine is in its usual 

 fractured condition, and some of the larger cracks have been filled 

 up with the fine crystallized matrix in which they are imbedded ; 

 there is no crowding of the particles ; in this case, therefore, the 

 olivine was not only crystallized, but fractured before the consolid- 

 ation of the mass. In another section of basalt the crystals of augite 

 and olivine are somewhat rounded, and the cracks filled up, so that 

 they probably existed as crystals or grains before the ejection of the 

 lava. 



One of the most important aids in the examination of rocks and 

 minerals is afforded by polarized light. In many cases it enables 

 us at once to discriminate between different minerals, and not un- 

 frequently affords clear evidence of changes which have taken place 

 subsequently to the consolidation of the substance under examination. 

 When a thin section of a crystal is placed on the stage of the micro- 

 scope, and a beam of polarized light passed through it, the beam is 

 depolarized, and generally exhibits colours due to interference ; the 

 intensity of colour varies according to the direction in which the 

 crystal is cut, and consequently in examining a section of rock, the 

 various sections of any one mineral do not always give the same 

 result ; but as the crystals of igneous rocks lie at all angles, it is 



