364 E. C. HARDER AND R. T. CHAMBERLIN 



Itacolumi quartzite. — The series of indurated rock formations 

 is capped, so far as the iron-ore district is concerned, by a thick 

 quartzite which is not very different Hthologically from the Caraga 

 quartzite. As this upper formation is perhaps displayed to best 

 advantage in the prominent peak of Itacolumi, the popular lookout 

 point near Ouro Preto, it may well take its name from that well- 

 known mountain. It is the youngest member of the regular sedi- 

 mentary series yet discovered in central Minas. 



While this formation where best exposed is predominantly 

 quartzite, it contains also a large proportion of schistose rocks. 

 The quartzite grades laterally into sericite, quartz, and talc schists, 

 so that, in places, the formation is largely composed of schist 

 with a few prominent and persistent layers of quartzite. This 

 variation in lithology is so characteristic both of the Piracicaba 

 formation and of the Itacolumi quartzite that the two may perhaps 

 well be regarded as one formation, being formed during a continuous 

 period of deposition, schists predominating in the lower part and 

 quartzite in the upper part. They strongly suggest delta deposi- 

 tion, individual layers being seen to grade from coarse quartzite 

 into argillaceous schist within a distance of 4 or 5 kilometers. 



In fact, there is much to indicate delta deposition in most of the 

 members of the Minas series. The Caraga quartzite, Itabira iron 

 formation, Piracicaba formation, and Itacolumi quartzite all 

 exhibit rapid lithological variations as their layers are traced 

 along the strike as well as across it. At the same time, they 

 show also in some places great thicknesses and extremely rapid 

 variations in thickness. These are not the results to expect if the 

 sediments were laid down in regular succession upon the relatively 

 flat and uniform ocean floor; but they are just what might be anti- 

 cipated if they represent the topset, foreset, and bottom-set beds 

 of delta formation. Under the former hypothesis it is not easy to 

 see how such great thicknesses of shallow water deposits could be 

 formed under any rational system of crustal dynamics; under 

 the hypothesis of inclined sedimentation the total thickness of 

 the series of shinghng layers laid down in shallow water may 

 very easily become as great as the strata of the Minas series 

 require. 



