386 A. C. LANE — PORPHYEITIC APPEARANCE OP ROCKS 



But should this always be true, and are porphyritic crystals always 

 intratelluric ? In answering this question * and questions of relative age 

 of formation of different minerals, a series of specimens taken at various 

 known distances from the margin will be of great value. That is what I 

 wish to show. 



Conceivable Classes of Phenocrysts 



It may safely be assumed as possible that the earth's interior is solid. 

 Being under great pressure, it will naturally be in the form of greatest 

 density, which is the crystalline. At times under certain conditions (I 

 presume usually relief of pressure in part) it liquefies or fuses, and then 

 later by restoration of pressure, loss of heat or water or gases (the agents 

 miner alizateurs of French writers) becomes solid again, and in the mean- 

 time may have flowed more or less freely, being erupted or intruded into 

 new surroundings. There is no reason why the process of liquefaction 

 and solidification, in whole or in part, may not be many times repeated. 



Now, the larger crystals or phenocrysts, which give the porphyritic 

 appearance, may be of at least five classes, diverse in origin : 



1. They may be relics of an early stage of slow consolidation within 

 the earth (they can hardly date back to the days of the "planetessi- 

 mals " ) — be unliquefied fragments of the rock which furnished the 

 magma. 



These have corroded outlines and are akin to Lacroix's f " enclaves 

 homoeogenes." 



The quartz phenocrysts of many porphyries, and the hornblende and 

 mica phenocrysts of many basalts, I should refer to this class. 



2. They may have been formed during an eruptive act which took 

 place within the range of the conditions (temperatures) of their crystal- 

 lization, so that they formed as they were borne floating along. This is 

 the origin of many porphyritic crystals of feldspar, with sharp outlines 

 and fluidal arrangement, either of themselves or of the flow lines of the 

 surrounding magma. It seems likel}^ that sharp zones of alternately 

 varying composition would be especially characteristic of this class, the 

 magma being changed slightly by mixing, while a gradual and uniform 

 change in the composition of a crystal might occur while the crystal 

 grew in a magma at rest. 



It is important to note, since it renders a confusion with another class 

 more likely to occur, that these rhyocr^^stals or floating crystals will 

 naturally be formed first near the margin, where its chilling effect is felt, 



* Asked already by L. V. Pirrson, Am. Jour. Sci., vii, 1899, p. 271. 

 fLes Enclaves des Roches Volcaniques, 189:i 



