342 



PHYSICAL GEOLOGY 



tions, it is often found that the sedimentary rocks are greatly altered 

 near the source of the heat (Figs. 334, 335). This is shown by a 

 change in color, in hardness, and in texture, and in some cases by 

 the development of new minerals. Bituminous coal is changed to 



anthracite coal, or 

 even in the most 

 extreme stage to 

 graphite ; limestone 

 is metamorphosed to 

 marble; soft sand- 

 stone may be con- 

 verted into hard 

 quartzite; and shale 

 may be metamor- 

 phosed to dense, 



Fig. 335. — Map showing the metamorphic zone (dotted) compact rocks, such 

 about an igneous intrusion. as schist and horn- 



fels, a compact flint- 

 like rock. The amount and extent of contact metamorphism depends 

 upon the amount of heat and to an important degree upon the gase- 

 ous emanations (mineralizers) given ofF by the molten rock. For 

 example, if molten rock is intruded into a narrow fissure, the surround- 

 ing rock will usually be little affected (Fig. 336), since the magma, 

 having a comparatively small amount of heat, soon loses it to the 

 neighboring rocks. Moreover, 

 the quantity of gas present is 

 too small to produce a marked 

 change. In the case of great 

 intrusions, however, such as 

 stocks or batholiths, the country 

 rock may be greatly altered 

 thousands of feet away. The 

 effect of an intrusion is naturally 

 greatest when the supply of heat 

 is large and long-continued. In 

 some cases, where fragments of 



the surrounding rock have been inclosed in the magma, 1 black shale 



has been baked to a hard, red, porcelain-like rock; granite has been 



more or less completely fused to dark green or black glass; and 



1 Powers, S., — The Origin of the Inclusions in Dikes, Jour. Geol., Vol. 23, 1915, pp. 1-10. 



Fig. 336. — Diagram showing the greater 

 metamorphic effect of an igneous intrusion 

 along bedding planes. 



