R. R. Walls — Diamond Pipes in Brazil. 447 



composition, and some of the specimens here described as rhyolites 

 show all the characteristics of lava-flows. 



From the mineralogical point of view, the most interesting features 

 of the later Archaean and Mesozoic igneous rocks is the dominance 

 of alkaline characters. The felspar is mainly perthite, while the 

 ferromagnesian minerals are chiefly riebeckite and aegirine ; the 

 magma was therefore rich in soda ; however, no nepheline has been 

 found in the more basic (syenitic types). 



Although tin, wolfram, and topaz are abundant in the area, and 

 fluorspar occurs locally, it is remarkable that among these specimens 

 not a single occurrence of tourmaline has been observed. It is evident 

 that the pneumatolytic phase was characterized by fluorine to the 

 exclusion of boron. 



The Existence, of Diamond-bearing " Pipes " in Brazil. 



By Robert R. Walls, M.A., B.Sc. 

 (PLATE VIII.) 



T|^HERE has been great controversy as to whether or not there 

 -*- exist in Brazil diamond-bearing " pipes " such as are found in 

 South Africa. That something of the kind should exist seems to be 

 pretty well acknowledged, but any evidence in support of the 

 discovery of a " pipe " has always been very meagre. The following 

 observations made by the writer on a few mines in the neighbourhood 

 of Diamantina, in the state of Minas Geraes, might be of interest 

 on this somewhat vexed question. 



The writer had no practical experience whatever of diamond 

 mines when he arrived in Brazil, and was only drawn into the subject 

 by accident. When studying the sandstones and quartzites of the 

 Serra do Espinhaco he found that in many places these had been 

 " stewed up " as it were and metamorphosed into rock crystal. 

 There was no sign of a granite or aplite intrusion in the neighbour- 

 hood, but these crystal formations all lay close to the great 

 diamond mine at Sopa, and from the field evidence alone it seemed 

 as if this latter must have been an igneous intrusion. 



The rocks of the district are all hard quartzites, remarkable for 

 the horizontality and regularity of their bedding. The great mine 

 takes the form of a fissure, a mile or more in length, and seems to 

 run down vertically through the bedded quartzites, although the 

 actual contact was nowhere exposed. This pipe or fissure is filled 

 with a sericitic schist crowded with small crystals and concretions 

 of iron-ores. It is green in colour, and so soft that it can be easily 

 broken down with wooden spades and bateas. This green earth 

 is then washed by negro labour, and the iron-ores and diamonds are 

 left as a heavy residue. The green earth bleaches to a white clay, 

 and is found all down the streams mingled with quartz sand. The 

 Sopa mine lies on the very summit of the ridge of the Serra 

 do Espinhaco, and the streams which run down from it are 



