898 A TREATISE ON METAMORPHISM. 



amount of ferric oxide is 6.25 per cent in clays and soils and 4.03 per cent 

 in the shales," it is clear that the small amount of carbon in the shales 

 furnishes, by its reducing action, an entirely adequate cause for the partial 

 change of the ferric to ferrous iron as metamorphism of the pelites con- 

 tinues from muds to shales, slates, schists, and gneisses. 



The fact that, with the exceptions of the chemical changes mentioned, 

 the compositions of the pelitic gneisses, schists, and slates, where unmodified 

 by igneous intrusions, correspond with those of the shales and muds is 

 positive evidence of the general conclusion given on pages 145, 764-766, 

 that circulating ground waters are unable to transport material from an 

 outside source through rocks the particles of which are so small as to 

 furnish only subcapillary openings. Of this principle the pelite family 

 affords the best illustration. 



The development of the minerals will first be considered at the stage 

 where the rocks are slates, and second where they have been transformed 

 to schists or gneisses. 



Development of minerals of slates. Olltof the COllStitUentS whidl are preSdlt ill 



the irresolvable background of the shales, innumerable flakes of mica and 

 small particles of quartz develop. The feldspar, with the addition of other 

 constituents, also largely alters into quartz and mica. (PI. XI, A.) From 

 a single fragmental grain of feldspar are produced many individuals of 

 interlocking quartz and mica. (PI. XI, B.) The minerals formed in the 

 interstices interlock with one another and with those produced from the 

 feldspar grains, so that an approximately uniform interlocking groundmass 

 of quartz and mica is produced. If fragmental grains of quartz are not 

 present to indicate the clastic character of the rock, the secondary inter- 

 locking quartz and mica do not betray this. The most abundant mica is 

 usually biotite. Frequently muscovite accompanies the biotite. The 

 sources of the material for the micas of the slates are the same as for the 

 micas of the schists and gneisses. (See pp. 899-900.) 



While in the ordinary slates the process of recrystallization as above 

 described is inaugurated, it is far from complete, and many of the original 

 mineral particles of the mud usually remain partly changed or wholly 

 unchanged. 



« Clarke, cit., Bull. 168, p. 17, col. C. 



