GEOLOGY OF CENTRAL MINAS GERAES, BRAZIL 367 



the series which might aid in determining its geologic horizon. 

 The strata have been metamorphosed to such an extent that fossil 

 forms might originally have been present in these beds and have 

 since been obliterated in the process of metamorphism. But to 

 what extent the absence of fossils is due to an original paucity of 

 impressions in the original sediments, and to what extent to 

 obliteration of existing forms by subsequent metamorphism we 

 cannot say. The age of the series therefore remains problematic. 

 It is simply known to rest unconformably upon the probable 

 Archean complex. The general nature of the strata harmonizes 

 well with the hypothesis that these beds are of pre-Cambrian age. 

 This is the age generally assigned to the series by the geologists 

 of the Brazilian Survey. At the present time we can hardly go 

 farther than to note the general parallelism between this Brazilian 

 metamorphic series and the Proterozoic sediments of other parts 

 of the world, especially the Lake Superior Algonkian series. The 

 heavy quartzites, schists, and iron formations indicate not very 

 widely different conditions in these two regions. But while the 

 sedimentary series in Minas presents a strange parallelism to the 

 Algonkian series of the Lake Superior region in North America, 

 and to various other pre-Cambrian terranes, no definite evidence 

 has yet been found to preclude its belonging in whole, or in part, 

 to the early Paleozoic.^ 



PRE-DEVONIAN DEPOEMATION 



Just when this long period of sedimentation in central Minas 

 was interrupted is not yet apparent. What is known is that it was 

 followed, whether shortly, or after a considerable interval, by a 

 period of great deformation. Mountain-making movements pro- 

 foundly affected a belt running across central Minas.^ The strata 

 were both folded and thrust-faulted on an extensive scale. At the 

 same time the rocks suffered metamorphic changes ; the sandstones 

 became quartzites, the shales were altered to the present schists, the 

 carbonate rocks were recrystallized, and the iron formation dehy- 

 drated to the hematite and itabirite which we see today. 



^ See J. C. Branner, Geologia Elementar, Rio de Janeiro, 1906, p. 217. 

 ^O. A. Derby, "The Serro do Espinhago, Brazil," Jour. Geo!., XIV (1906), 

 347-401- 



