312 SUMMAEY OF CURRENT RESEARCHES RELATING TO 



crystallization is proceeding slowly. They were formed in the molten 

 magma when it was still inclosed in the subterranean reservoirs. This 

 slow growth is clearly proved by the formation of the crystals in con- 

 centric zones, and by their size. These large crystals, existing ready 

 formed in the lava at the time of its eruption, are surrounded by micro- 

 lites or by a vitreous mass. It was after their slow development in 

 the magma, during an intra-telluric period, that the mass in which 

 they floated was upraised. The period of calm was succeeded by 

 one of agitation, and the lava in its violent ejection carried forth the 

 crystals, breaking them, corroding them, and partially fusing them. The 

 Microscope offers distinct evidence of these phenomena. We see the 

 large crystals dislocated and their fragments dispersed, their edges 

 rounded and eroded, and their substance invaded and penetrated by the 

 paste. 



While the physical and chemical agencies brought into play by the 

 movement of the lava thus attack the ancient crystals to the verge of 

 demolition, the microlites are in course of formation. This vitreous 

 matter, in which the large crystals float, solidifies as a mass of micro- 

 scopic individuals. The latter are therefore related to a second phase 

 of crystallization : they are developed in a moving viscous magma, and 

 their further growth is arrested by the rapid cooling which induces 

 solidification en masse. 



The fluidal arrangement of the microlites distinctly shows, too, that 

 the crystalline action was contemporaneous with the movement of the 

 lava-flow. Indeed, we see in microscopic preparations that the micro- 

 lites are accumulated around the large sections of crystals, forming 

 wavy trains and presenting the arrangement which micrographers 

 designate as fluidal structure. It is marked by the orientation of these 

 infinitely small acicular crystals. When these streams of microlites 

 meet the large imbedded crystals, they sweep round them, crowding 

 into the spaces between the large sections, accommodating their flow to 

 these outlines, and preserving for us the last movement of the mass at 

 the very moment of solidification. 



The Microscope, therefore, proves that crystallization in lavas belongs 

 to two periods : the fi.rst, anterior to the eruption, during which the 

 large crystals already found are suspended in a mass that we may regard 

 as entirely vitreous ; and the second period, when the microlites and 

 embryonic crystalline forms are separated, dating from the ejection or 

 outflow, and contemporaneous with the solidification of the rock. 



From these microscopic observations on the crystals of the second 

 period, we may conclude that they are formed purely and simply by 

 igneous action, without requiring the hypothetical temperatures and 

 pressures formerly considered necessary, and without that absolute 

 repose regarded as needful for the regular crystallization of minerals. 

 We see, indeed, that the microlites are formed after the outflow, at the 

 normal barometric pressure and at a temperature far from being as high 

 as generally supposed, and we witness the births of the crystals during 

 the very flow of the lava stream. When the cooling is extremely rapid, 

 the microlites have no time to form, and the lava can produce only 

 crystallites. 



But the Microscope enables us to determine the chronology of the 

 crystals in lava in a still more detailed manner. We have already 

 distinguished two great periods in their history ; let us now indicate in 



