BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 



Vol. 14, pp. 385-406 September 30, 1903 



PORPHYRITIC APPEARANCE OF ROCKS ^ 



BY ALFRED C. LANE 



(Read before the Society December 31, 1902') 

 CONTENTS 



Page 



Object of paper 385 



Conceivable classes of phenocrysts 386 



Application of the theory of cooling to question of grain 389 



Marginal temperature kept constant 390 



Case of the contact zone 391 



Approximate formulae 395 



Eocrystals or phenocrysts formed soon after rest 397 



Border phenocrysts or oriocrystals 400 



Medford dike, Massachusetts 400 



Marginal phenocrysts 401 



Mathematical summary 402 



Object of Paper 



The variation of the texture of igneous rocks as the margin is ap- 

 proached has long been recognized, and references to marginal and 

 " Rand facies " phenomena are not infrequent. Yet generally they lack 

 exactness, for in but few cases have the exact distances of specimens of 

 an igneous rock from the margin been noted. Yet a little thought would 

 lead one to expect important aid to the petrographer who wants to know 

 the origin of things from studying just these marginal sections, and espe- 

 cially a series at known distances from the margin, since every one knows 

 what a difference has been made in paleontology by the studies of suc- 

 cessive stages in the development of a form, and we ma}' find similar 

 light at the margin of an igneous rock on the different generations of 

 crystals and phenocrysts. For instance, if, as Michel-Levy says, there 

 are two generations even in granitic rocks f only with similar products, 

 we may perhaps find that toward the margin the younger generation 

 grows finer and the texture will become plainly porphyritic. 



♦Plates referred to are those of previous article "Studies of the grain of igneous intrusives." 

 t Structures et Classifications des Roches Eruptives, 1889, p. 29. 



LIV— Bull. Geol. Soc. Am., Vol. 14, 1902 (385) 



