218 C jSF. Ferrner — Crystallization of a Basaltic Magma 



many modifications of structure resulted. The chief effect, as 

 regards the features which will be considered in this paper, 

 was that the fused magma was rapidly chilled and rendered 

 viscous. In places the stiffening liquid accumulated in masses 

 of rounded or bowlder-like forms simijar to the "pahoehoe" of 

 Hawaiian flows. The progress of crystallization was checked 

 at various stages and the bowlder-forms were crusted with 

 basaltic glass, in which examination with the microscope shows 

 few phenocrysts, while the more slowly cooling interiors of the 

 bowlders assumed the normal texture. Between the two types 

 transitions occur, by which one may trace the passage of the 

 microlites of the vitrophyrs into the well-developed crystals of 

 normal basalt. 



The glass-encrusted bowlders are especially well developed 

 in an area lying between Paterson and Montclair Heights. 

 Quarries have been opened at several points, as it has been 

 found that this variety of trap is easily blasted and crushed 

 for road-material. It has therefore been possible to obtain 

 material for petrographic study unacted upon by weathering. 



Scope of the Article. 



Two principal features of the Watchung basalt will be con- 

 sidered, and in accordance with this the article is divided into 

 two parts. Fart I will deal with the order of crystallization of 

 the constituent minerals, and it will be shown that, contrary to 

 certain prevalent ideas regarding the order of succession of the 

 minerals of a basaltic rock, the three constituents, plagioclase, 

 diopside, and magnetite, began to appear almost simultane- 

 ously from the magma and continued to crystallize side by side 

 until complete solidification was attained. In order to show 

 that this result is demanded by the laws of crystallizing solu- 

 tions, a very brief outline of these laws will be given as a pre- 

 liminary to the petrographic study. 



In Part II resorption-phenomena will be considered in con- 

 nection with the resorption of olivine, a minor feature as 

 regards the constitution of the rock, but very significant in its 

 interpretation. 



The law of mass-action will be considered in this connection, 

 and the author will endeavor to show that the usual explanation 

 of resorption is inadequate to explain certain phases of the 

 phenomenon, but that a very satisfactory explanation may be 

 derived from an application of van't Hoff's law. 



Part I. 



Process of Crystallization as demanded by the Laws governing 

 Eutectiferous Solutions. 



In order that the crystallization of a magma may proceed 

 strictly along the lines indicated by eutectic laws, it is essential that 



