352 0. A. Derby — Association of Argillaceous 



the material of the supposed dikes is characterized, with the 

 single exception of the Dattas rock, by the mineral monazite 

 which, so far as at present known, may be taken as peculiarly 

 characteristic of granitic magmas. The original dikes may 

 therefore be presumed to have been acid porphyries, or basic 

 phases of such porphyries. If, however, this conclusion is 

 correct, all eruptives of whatever nature existing in the series 

 previous to the uplifting and shearing must have suffered a 

 similar change and afforded planes of weak strata favorable to 

 the subsequent injection of pegmatites, or of other types of 

 eruptives. In the overlying series to which the conglomeritic 

 diamond-bearing deposits examined belong, the quartz veins 

 observed show none of these characteristics and appear to be 

 ordinary veins of segregation. The same is also the case with 

 the very numerous and large veins observed along about fifty 

 miles of road over a newer series of graywackes, slates and 

 limestones in the region lying immediately to the west of the 

 gold and diamond section of the Serra do Espinhaco. On the 

 other hand, a number of scattered observations indicate that 

 the veins of the gold-mining districts of the Serra do Espinhaco 

 present characteristics similar to those of the diamond district 

 and difficultly reconcilable with the current hypotheses of a 

 purely segregational origin. In the only one that has been 

 critically examined, that of Passage m near Ouro Preto, Dr. 

 Hussak, as already referred, has found characteristic granitic 

 accessories and contact minerals in a great sulphuret vein, and 

 as the somewhat similar veins of the Pary and Morro Yelho 

 mines carry garnets in the one case and microscopic zircons in 

 the other, it may be suspected that they will on close examina- 

 tion, for which material is unfortunately not at hand, give a 

 similar result. Dr. Hussak has also shown in his various papers 

 in the Mineralogical Magazine, on the remarkable association 

 of minerals in the cinnabar-bearing gravels of Tripuhy, that 

 these carry monazite and xenotime, the latter of the same type 

 as in the Diamantina district, along with the curious titano- 

 antimoniates of which one has been traced to a sericitic schist 

 very similar in appearance to that above described, and to a 

 pyritiferous sericitic schist found in place in the immediate 

 vicinity in which rare prismatic fragments of monazite of the 

 Sopa type were detected. Lithomarge with other peculiar 

 clays are, with quartz, characteristic features of the topaz and 

 euclase-bearing veins near Ouro Preto, and lithomarge is a 

 constant accompaniment of the peculiar gold and iron-bearing 

 jacutinga veins (?) in the iron ore beds of the same region. 

 Thus the association of argillaceous materials with quartz veins 

 is wide spread and varied in its occurrence, and the observa- 

 tions here recorded are suggestive of the necessity of a careful 

 revision of the current hypothesis regarding the segregational 

 origin of many of these veins. 



