646 A TREATISE ON METAMORPHISM. 



Probably the most common of the various segregations is that of 

 silica. This perhaps most frequentl}- takes place in limestones, pro- 

 ducing chert or quartz masses. According to Hinde," silicification has 

 extensively occurred at Spitzbergen and Axels Island, where formations 

 once largely carbonates, aggregating 250 or more meters in thickness, are 

 composed almost entirely of chert. 



Cases of extensive serpentinization are those of the metamorphosed 

 sandstones and igneous rocks in the Coast Range of California, described 

 by Becker, where serpentine is said to replace quartz, feldspar, and apatite 

 on an extensive scale. 6 



IGNEOUS WORK. 



The igneous work of the belt of cementation is all comprised under 

 "injection." 



INJECTION. 



By injection is meant the penetration of a rock by a molten magma. 

 In the belt of cementation the injecting rocks make their way chiefly by 

 following fractures, such as faults, joints, bedding planes, fissility planes, 

 irregular fractures of brecciation, etc, The injected material therefore fills 

 the larger crevices, such as those produced' by faulting, jointing, bedding, 

 fissility, or brecciation, and to some extent it may penetrate the interspaces 

 between the individual grains — for instance, those of the ordinary sedi- 

 mentary rocks. But it is probable that pure ig'iieous injection between the 

 mineral particles of the dense rocks does not usually penetrate a great dis- 

 tance from a continuous mass of the magma. Not only do the injecting 

 masses utilize the openings already formed, but they force the walls apart 

 and extend the openings, thus making possible intrusive masses of large size. 

 The intrusive masses vary in size from great laccoliths many kilometers in 

 extent and thousands of meters in thickness, through numerous great dikes 

 or sills, man} r of which may be a hundred or more meters in thickness and 

 many kilometers in extent, through dikes and sills of small size, to minute 

 laminae between the fissile leaves or to stringers between the grains. 



'< Hinde, (7. J. , On the chert arid siliceous schists of the Permo-Carboniferous strata of Spitzbergen : 

 Geol. Mag., new ser., dec. 3, vol. 5, 1888, pp. 241-251. Reviewed in Am. Jour. Sci., 3d ser., vol. 36, 

 1888, p. 73. 



b Becker, G. F., Geology of the quicksilver deposits of the Pacific slope: Mon. U. S. Geol. Survey, 

 vol. 13, 1888, pp. 122-125. 



