GRANITIC ROCKS. 115 



essentially coterminous, these variations are of sufficient extent to be indicated 

 when the mapping of their areas is undertaken. All about the northern mar- 

 gin, particularly near the Norfolk County Basin, where the red Carbonifer- 

 ous sediments abound, the feldspar of the granitite is in many localities of a 

 deep-red color. The association of these red feldspars with the red Carbonif- 

 erous rocks along this northern border of the field maybe without significance, 

 but the relation is noteworthy. The coloration of the feldspar, however, 

 has arisen apparently through the penetration of an iron oxide in solution, 

 following upon atmospheric decay. The oxidation of the iron-bearing sili- 

 cates in the granitite, e. g., the biotite and the hornblende, may have fur- 

 nished the iron, in which case there is no reason for supposing that the 

 coloring matter has leached downward from the formerly overlyino- red 

 Carboniferous beds. On the contrary, it is more probable that the red beds 

 owe their color to this iron oxide having been set free in pre-Carboniferous 

 times. The surface exposures of this red granitite are simply the underlying 

 base of the zone of decayed rock which was swept off in the making of 

 the Carboniferous sediments. The objection which may be raised to this 

 view is that the granitite has been at least twice exposed to atmospheric 

 decay, once in pre-Carboniferous times, and again in recent geological 

 times, and that the red color of the feldspars to-day may be due to recent 

 rather than to ancient decay. But the arkose at the base of the Carbonif- 

 erous is sufficient proof of the ancient period of atmospheric decay, and 

 the immediately overlying red beds show that the process of rock discolor- 

 ation by percolating iron oxides went on in early Carboniferous time 

 essentially in the manner advocated by Russell. 



The possibility of the coloration of sediments, at least locally, by this 

 means is attested by observations which were made during the present 

 survey. At oue point in the region of red granitite I was struck with 

 the redness of the water standing in the bottom of a small test quarry, 

 and with the film of red mud deposited everywhere beneath the water 

 level. There can be no doubt, therefore, that the red granitites are at 

 the present time capable of staining sediments with a red iron oxide film. 

 This process seems to be a local instance contrary to the general yellowish 

 and brownish hues so widely noted as the result of decomposition of rocks 

 in this latitude. 1 



1 W. O. Crosby, Am. Geologist, Vol. VIII, 1891, pp. 72-82. 



