F. H. Lahee — Metamorphism and Geological Structure. 453 



central region,* If, on the other hand, crystallization were 

 initiated before the cessation of injection, the advancing 

 magma might grow more acid by selective formation of the 

 more basic constituents along the walls. In this manner, a 

 dike, composed of quartz, feldspar, etc., might consist of quartz 

 only at its outer end. 



We may add further that in pegmatites or quartz veins of 

 which the crystallization has been thus controlled by extrane- 

 ous water, the relative amount of such minerals as topaz, tour- 

 maline, and fluorite, should be small. 



As for the metamorphic effects ascribed to pegmatites, the 

 investigations of many geologists have demonstrated that 

 granite intrusions have often, although not always, produced 

 considerable exomorphic alteration.! The changes are due 

 partly to heat and partly to the action of escaping volatile con- 

 stituents.^: The effects include induration, recrystallization, 

 lit-par-lit injection, and metasomatism. At a distance, recrys- 

 tallization in impure elastics may be evidenced by the presence 

 of rutile needles ; nearer the intrusive, by the occurrence of 

 'knots' and the micas ; and still nearer, by such minerals as 

 feldspar, andalusite, garnet, and amphibole, and by the aggrega- 

 tion of siliceous matter into large irregular quartz grains. 



Concerning the relative importance of heat and volatile con- 

 stituents in producing contact metamorphism, Geikie writes, 

 "It would appear that mere dry heat produces only a small 



amount of chemical alteration." Also, "the presence of 



pneumatolytic agents .... has been largely influential, com- 

 bined, doubtless, with great pressure, high temperature, and a 

 continuance of these conditions for vast periods of time."§ 

 The French school lays much stress upon the importance of 

 metasomatic processes due to the action of gases. || Harker 



* This type of distribution of the constituents, according to which the 

 more basic minerals are marginal and the more acid ones are central, has 

 been described by Smith and Calkins (Smith, G. O., and Calkins, F. C, A 

 Geological Reconnaissance across the Cascade Range near the 40th Parallel. 

 U. S. G. S., Bull. 235, 1904, p. 76), H. H. Reusch (Die Fossilien Fuhrenden 

 Krystallinischen Schiefer von Bergen in Norwegen. German trans, by R. 

 Baldauf, Leipzig, 1883), and many others. But Watson notes dikes in which 

 quartz and feldspar are marginal, and mica, tourmaline, and garnet are con- 

 centrated in the middle zone (Watson, T. L., On the Occurrence of Aplite. 

 Pegmatite, and Tourmaline Bunches in the Stone Mountain Granite of 

 Georgia, Jour. Geol., x, p. 186, 1902). 



f Geikie, A., op. cit., pp. 726, 778 et seq. 



JG. W. Hawes notes the addition of boric acid (this Journal, xxi, p. 21, 

 1881). Certain volatile components, such as water gas and carbon dioxide, 

 may be driven out of minerals in the country rock by the heat and may then 

 play an active part in the metamorphism. (Van Hise, C. R., op. cit., p. 713). 



§ Geikie, A., op. cit., p. 767. 



|| See Harker, A. (op.' cit., p. 305), for a summary of the French views, 

 and for several references. Michel-Levy and Lacroix are noted. 



Am. Jour. Sci.— Fourth Series, Vol. XXXIII, No. 197.— May, 1912. 

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