BRANNER-AGASSIZ EXPEDITION TO BRAZIL I9I 



Aymores, at this particular place the heart of the Serra is rep- 

 resented by only a few isolated peaks that rose above the water 

 as islands when the sediments were being deposited. The route 

 followed by the railway is, therefore, a sort of gap in the range, 

 where the ancient serra is represented only by these tall peaks. 



From kilometer i6o, where the railway descends the west 

 slope of the Serra, the exposures of crystalline rocks become 

 more and more abundant until, in the narrow valley from 

 the watershed to the Rio Mucury, the Eocene (?) beds dis- 

 appear entirely, and the hills on either side are all of crystalline 

 rocks, which continue nearly to the city of Theophilo Ottoni. 

 Professor H. W. Turner, of the U. S. Geological Survey, has 

 kindly examined the rocks collected on this trip ; the one found 

 at this place he calls qtiartz-monzojitte, adding that it is a rock 

 that most petrographers would call a gabbro.^ 



At kilometer 171 the railway reaches the banks of the Rio 

 Mucury. This stream throughout nearly its entire length flows 

 over a bed of coarsely crystalline rocks, mostly of gabbro, or 

 quartz-monzonite. The accompanying illustration (fig. 7) 

 shows the round, exfoliated peak near the watering station, Sete 

 de Setembro, near kilometer 172. This peak itself was not vis- 

 ited, but the many exposures along the railway near its base are 

 all of quartz-monzonite. 



The immediate valley of the Mucury is here hardly more 

 than half a kilometer wide in the bottoms, and the hills rise on 

 both sides to a height of a hundred meters or less. Through 

 its entire length this valley remains narrow and its sides are 

 steep and heavily wooded ; only near its head-waters do the 

 valleys widen out somewhat. At kilometer 188 the railway 

 crosses to the right side of the river, upon which it continues the 



1 The following is Mr. Turner's description : 



F- 93- "Quartz-monzonite. Macroscopically a medium-grained, dark, 

 slightly greenish granolite. 



"Microscopically composed of plagioclase >■ orthoclase > ^ quartz > ^ 

 rhombic pyroxene p> biotite. There are present as accessories iron oxide, apatite 

 and zenotime. The plagioclase, quartz, and pyroxene occur in comparatively 

 small anhedrons, in part inclosed in large untwinned orthoclase grains which 

 show minute interpositions, forming the ' faserige ' orthoclase of Becke, The 

 biotite is deep reddish-brown in color, and frequently all the foils in the field 

 extinguish at once, although disconnected in the section." 



