490 A TREATISE ON METAMORPHISM. 



Of these metatropic changes b} r the action of dry heat under pressure that of the 

 formation of marble has been experimentally verified years ago by G. Rose and more 

 recently by Richthofen and others/' 



Usually all of the direct dry heat contact effects are very limited in 

 amount, extending only short distances from the igneous rocks. Ordinarily 

 the direct effect is modified by indirect effects In many of the cases men- 

 tioned by Irving, where the material when modified was not in the belt of 

 weathering, it is highly probable that the results were not accomplished by 

 dry heat alone, but with the assistance of gases and vapors. Commonl}^ 

 the rocks which are being baked, fritted, or fused are in the midst of gases 

 and vapors, including, of course, abundant water vapor and oxygen. Also, 

 as in the cases mentioned by Irving, in many instances which have been 

 given of the effect of dry heat alone the material when altered was below 

 the belt of weathering; but as the different cases of metamorphism attrib- 

 uted to dry heat have not been studied in reference to the different zones 

 and belts of metamorphism, and it is therefore impossible to discriminate 

 with certainty the cases which are truly due to dry heat alone from those 

 in which heat works in conjunction with gases and vapors, or even in con- 

 junction with water, in so far as the gases and vapors are present, we have 

 the indirect effects considered below mingled with the direct contact effects. 



INDIRECT CONTACT EFFECT, OK WORK OF FUMAROLES AND SOLFATARAS. 



It has been said that the indirect contact action is due to gaseous 

 solutions. The action of such solutions may extend much farther than the 

 direct contact effect. The belt of weathering may be permeated locally 

 with hot gaseous solutions. The work of these gaseous solutions is 

 essentially of the same nature as that of ordinary gaseous solutions, 

 discussed on the previous pages (see pp. 59-63) ; but the gaseous solutions 

 adjacent to igneous rocks usually contain a greater quantity of the active 

 chemical agents than do ordinary solutions; and, moreover, their tempera- 

 ture is much higher than normal. This gives a combination of conditions 

 which results in much more rapid alteration than the average of the belt of 

 weathering and alteration of a different kind. The exceptional regions in 

 which these unusual conditions obtain are A^olcanic In volcanic regions 



" Irving, A., Chemical and physical studies in the metamorphism of rocks, Longmans, Green & Co., 

 London, 1889, p. 76. 



