ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 311 



chemical analysis generally gave merely the composition of the total 

 rock, and its mineralogical composition was only suspected. The 

 intimate texture of the rock remained impenetrable ; it was impossible 

 to determine with certainty the order in which the constituents of the 

 molten mass had solidified ; neither could we trace the various states 

 through which the crystals had passed — their germs, primordial forms, 

 and skeletons — or the aspect of the rock at different stages of its 

 development. 



Let us now apply the Microscope to the examination of a thin slice 

 of lava, rendered transparent by polishing. The lavas, as we have 

 said, may be compared to vitreous masses ; but whilst in our artificial 

 glasses we seek to obtain a pellucid and homogeneous product, the lique- 

 fied matter of volcanoes, when it flows forth, already contains certain 

 differentiated products. The glass which contains these bodies may be 

 regarded as the residue of the crystallization, whence the numerous 

 crystalline individuals have extracted their constituent elements. In 

 the black, brilliant, volcanic glasses, apparently opaque and destitute of 

 crystallization, the Microscope discovers a world of mineral forms. It 

 shows us their various states of growth, and the arrest of their develop- 

 ment, consequent on the more or less rapid consolidation of the mass. 

 It is especially in those rocks which, like obsidian, have preserved 

 almost wholly their vitreous character, and are homogeneous to the naked 

 eye, that we find the rudimentary crystals of curious form, representing 

 the first step in the passage of the amorphous matter to the crystalline 

 condition. Owing to the rapidity with which the vitreous paste consoli- 

 dated, the crystals were unable to grow, and their development was sharply 

 arrested. Hence the origin of these embryonic crystals which abound in 

 natural glasses, and which we designate as crystallites. Analogous crystal- 

 lites are produced in blast furnace slags, which have close relations to the 

 matter of lavas. Their common origin is betrayed by certain family 

 likenesses which the Microscope reveals. The slags, examined in thin 

 sections, exhibit rudimentary crystalline forms, similar to the crystal- 

 lites of volcanic glasses. 



But usually the crystals have not 'remained in this embryonic state. 

 If the lava has not been too rapidly cooled, the molecular movements are 

 retained, even in a semi-liquid mass, and the paste developes crystals of 

 minute dimensions, called microlites. These microscopic crystals are 

 formed in the heart of the vitreous magma during its slow consolidation. 

 Notwithstanding their infinite minuteness, these small polyhedra exhibit 

 with marvellous exactitude all their specific characteristics, such as we 

 are familar with in much larger crystals, and which we should not 

 expect to find in lavas. They often form by their interlacement a 

 beautiful network in the microlitic structure. 



The dimensions of these microlites, invariably microscopic, and 

 their arrangement, prove that they may be referred to a period of dis- 

 turbance ; that they were formed, indeed, at a time when the lava, 

 though still in motion, was solidifying. They separated from the magma 

 during the very act of outflow or eruption. 



Besides these microscopic crystals and these groups of crystallites, 

 which belong to the last stage of consolidation, the lava contains also a 

 supply of larger crystals, more fully developed, and in many cases 

 recognizable by the naked eye. These have been formed under calmer 

 conditions, analogous to those presented by a tranquil fluid in which 



