388 A. C. LANE — PORPHYKITIC APPEARANX'P: OF ROCKS 



these terms can be turned into brotocrystals, rhyocrystals,* eocrystals, 

 oriocrystals, and metacrystals respectively. 



The discrimination of these classes is important from the point of view 

 of genesis, and it is eas}^ to see that sections at the margin where the 

 development was arrested might be preeminently instructive, especially 

 in comparison with a series of sections toward the center. The food 

 crystals will be larger, sharper, and less corroded than at the center. 

 Flow crystals will be there if anywhere, and may be nowhere else, and 

 though they ma}^ be smaller than they are farther from the margin, if 

 group 2 blends with group 3, the difference between them and the ground 

 mass will ordinarily be more marked. Along near the margin the fric- 

 tion of the walls will produce eddies and send in cooler currents into the 

 mass, so that porphyritic flow crj^stals may be produced of which the 

 center of the rock mass may be entirel}^ destitute, the temperature hav- 

 ing remained too high for any crystallization until well after the magma 

 had come to rest. 



When the magma comes to rest, relatively cool spots produced either 

 by flow currents or the absorption of fragments will, if they are not 

 thereby chilled to below the crystallization point of the mineral which 

 we are studying, be points of extra slow cooling and extra coarse grain. 



Border crystals, oriocrystals, will most differ in size from the other 

 constituents, and the difference between them and the flow crystals is 

 that they will grow smaller toward the center. 



The third class, the early crystals, will be least conspicuous toward the 

 margin, most so toward the center. Ordinarily we hardly class them as 

 phenocrysts. 



As I have remarked, the formation of a mineral may occur during the 

 epochs characteristic of the second and third classes, or a core of the 

 first class may be built on by the latter. In such cases the distinctions 

 made will be blurred, though it is often not impossible to distinguish 

 the parts formed in different stages of growth. Such cases must be 

 treated individually. 



All the phenocrysts except the early formed and the metamorphic, 

 which are not ordinarily classed as such, will be best developed near the 

 margin. While arrangement in glomeroporphyritic aggregates or envel- 

 opment by flow lines are distinctive of the first two classes, corroded out- 

 lines and magmatic reaction rims are not infallible signs of the first class, 

 for, as I have shown for the olivine of the Keweenawan traps, during the 

 l)rocess of solidification there may be a concentration of certain con- 

 stituents of the magma which may lead to an attack of the earlier formed 



*F, E. Wright. 



