38 PHYSICAL AND CHEMICAL PBOPEETIES OP ROCKS 



and they take on large and well-developed forms. After a time, 

 owing to changed conditions, their growth is stopped, and the 

 rock solidifies with a new crop of smaller and less perfectly 

 developed forms. It is customary to speak of such a mineral 

 as occurring in crystals of two generations. In the case above 

 described, the first developed form the porphyritic constituents, 

 the phenocrysfs, while the latter formed are a part of the ground- 

 mass. Vitreous or glassy rocks may show, under the microscope, 

 minute, hair-like or rod-shaped forms, representing the first 

 stages of crystallization, but in which the process was arrested 

 before they were sufficiently developed to render possible an 

 accurate determination of their mineral nature. Such are termed 

 crystalUtes; those in drop-shaped or globular forms being called 

 gloliihtes, the rod-shaped ones helonites, and the twisted, hair- 

 like forms tr%ch%tes. 



The wide variation in mierostructure in rocks of essentially 

 the same chemical composition, but which have cooled under 

 the varying conditions indicated above, is shown in Figs. 1 to 

 4 of PL 5, Fig. 1 being a holocrystalline type, and Fig- 4 one 

 almost completely glassy, the first being a deep-seated rock, and 

 the last a surface lava flow. Intermediate structures are often 

 produced through a beginning of crystallization at certain pe- 

 riods, after which, and while a portion of the magma was still 

 fluid, it was brought under such conditions as resulted in a 

 more rapid cooling, the final result being a glassy, or micro- 

 crystalline rock with scattering porphyritic crystals, or pheno- 

 crysts. It has in many instances happened that, subsequent to 

 the formation of these earliest products of crystallization, a 

 second elevation of temperatures has taken place whereby the 

 magma has eaten into or corroded them, as is the case with 

 the quartz crystal shown in the centre of Fig. 3 of PL 5. 



Inasmuch as this study by the microscope involves the prepa- 

 ration of thin sections, a brief description of the methods pur- 

 sued may well be given here. The fact that a chip of rock, 

 however dense, can, without breaking, be ground so thin as 

 to be transparent, may at first seem strange, but in reality it 

 is readily accomplished. The work requires only patience and 

 the skill which comes from practice. A small chip of rock, 

 about the size of a nickel five-cent piece, is broken off with a 

 hammer, care being taken to get it as thin as possible without 

 fracturing. One side of this is then ground flat and smooth by 



