TYPES OF PRISMATIC STRUCTURE IN IGNEOUS ROCKS 1 



ROBERT B. SOSMAN 

 Geophysical Laboratory, Carnegie Institution of Washington 



The question of the cause of columnar or prismatic jointing in 

 igneous rocks was thought to have been satisfactorily settled by the 

 writings of Thomson, Mallet, Bonney, Iddings. and others, until it 

 was reopened recently by the investigations of several French 

 physicists. As the subject seems to be in need of further discussion 

 and experimental study, I have brought together observations on 

 several hypotheses of prismatic jointing, hoping to show that 

 the study of these structures may yield much more precise 

 information than is now available as to the original conditions of 

 occurrence of the igneous rocks in which such structures are found. 



CRYSTALLIZATION HYPOTHESIS 



The first hypothesis as to the origin of prismatic structure which 

 had any experimental or observational basis was that of Gregory 

 Watt, 2 and may be entitled the "crystallization hypothesis." 

 Watt, in 1804, observed that a large mass of basalt which he had 

 melted down in a reverbatory furnace crystallized radially from 

 centers which were fairly regularly spaced in a horizontal plane; 

 the intersections of these radially growing fibrous bundles formed a 

 network of hexagonal partings through the mass, leading Watt to 

 the conclusion that this manner of crystallization, by its vertical 

 extension upward from the base of a mass of basalt, must have been 

 the cause of the prisms found in the Giant's Causeway, Fingal's 

 Cave, and elsewhere. 



1 Presented before the Geological Society of Washington, April 28, 1915. 



2 Gregory Watt, " Observations on Basalt, and on the Transition from the Vitreous 

 to the Stony Texture," etc., Phil Trans., 1804, pp. 279-314. Watt also explains 

 clearly the contractional origin of such structures as mud and starch prisms. 



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