100 DR. J. W. EVANS ON THE [Feb. 1 8 94, 



IX. Igneous Rocks 



I have already referred to the ancient igneous rocks of Corumbii. 

 Von den Steinen found a granitic zone on the Xingu, 10° lat. S., 

 "which may, perhaps, be of similar antiquity, (6) p. 133. 



I met with an interesting augite-syenite at the Pao d'Assucar. 

 This, as its name implies, is a sugar-loaf shaped eminence on the 

 eastern bank of the Paraguay, about 21° 25' lat. S. It is one of a 

 group of hills rising out of the alluvial plain on either side of the 

 river. 1 The rock contains orthoclase somewhat altered, a little 

 plagioclase, brownish-green hornblende, green augite, biotite, abun- 

 dant sphene (in fairly large crystals visible to the naked eye), and 

 apatite. As is sometimes the case in augite-syenites, the biotite may 

 often be seen surrounding the augite. The sphene is occasionally 

 formed round the hornblende as a nucleus. 



Mr. Orville A. Derby, to whom I sent a fragment of this rock, 

 tells me that it closely resembles certain augite-syenites associated 

 with the nephcline (el8eolite)-syenites of Eastern Erazil, which he 

 has described, (15) p. 457, (16) p. 311, and (17) p. 251. I could not 

 detect elseolite in thin sections of the rock, though it shows some 

 gelatinization on treatment with acid ; possibly there may be some 

 too small to be recognizable, or such small particles as once existed 

 may have passed into some alteration-product. A nepheline-basalt 

 is reported from Paraguay at no great distance to the south, (14) 

 p. 247. 



About G kilometres (3| miles) south of the Pao dAssucar is another 

 exposure of somewhat similar rock. This is much altered, with 

 secondary quartz ; plagioclase-felspar is predominant. 



Mr. Derby has shown that the ekeolite and augite-syenites of 

 Eastern Brazil are either late Carboniferous or post-Carboniferous. 

 The Pao d'Assucar rock will prove to be of the same age, if, as is 

 believed, it is a distant representative of this important petro- 

 graphical province of nepheline-bearing rocks, which extends in the 

 opposite direction as far as the island of Sao Eernando da Noronha. 



If, as seems likely, the basalt-like rocks of South-eastern Matto 

 Grosso are identical with the widespread augite-porphyrites on the 

 other side of the Parana, they will be at least post-Carboniferous, 

 and probably of still later age. 



The Tapirapuam rock is ■ a rather coarse basalt, approaching a 

 dolerite. It contained abundant olivine, now altered into serpen- 

 tine and ferric oxide. Many of the felspars are ophitically included 

 in the larger augites. Most, however, occur among granular augite, 

 or embedded in opaque material consisting largely of magnetite, 

 ilmenite, and leucoxene. There is no evidence of the age of this 



1 The ' closing of the hills ' upon the river at this place has suggested the 

 name, Fecho dos Morros. 



