Diamond Regions of Eastern Minas Qeraes, Brazil. 211 



Neither of these hypotheses, however, will apply to Nos. Ill 

 and VI, which, if original elastics, must have been almost 

 purely micaceous sediments with, in the case of the latter, a 

 slight admixture of kaolin. No. Ill is certainly clastic as proved 

 by the heavy residue and it is presumed to represent a thin, 

 shaly parting inaconglomeritic quartzite derived from an under- 

 lying micaceous quartzite and for this such an hypothesis is not 

 improbable. Nos. II, VI and VIII also, if clastic, must like- 

 wise have been essentially micaceous but heavily charged with 

 limonite. It is, however, improbable that in five representa- 

 tives, taken by chance, of original argillaceous sediments, all 

 should prove to be essentially micaceous and none essentially 

 kaolinitic, and the hypothesis may be suggested that in the 

 case of marine clays a gain in alkalies may possibly take place 

 from soluble salts imprisoned from the sea water. No. I, 

 regarded as a clastic, must have been a singular mixture of 

 kaolinitic and magnesian clays. 



On the other hand, the hypothesis of eruptive origin for any 

 of these rocks, with the possible exception of Nos. V and VII, 

 involves that of unknown and improbable types, or that of an 

 important loss of lime and magnesia with a consequent concen- 

 tration of alumina, iron and alkalies. In the paper above cited 

 arguments for the possible eruptive origin of Nos. I and II 

 were deduced from the presence of autigenetic monazite, the 

 lack of recognizable allothigenetic elements and, in the case of 

 No. II, from a presumed connection with a decomposed sheared 

 dike in the immediate vicinity as well as of traces of original 

 structure. The first argument has proved fallacious but the 

 others still hold good and the second one is equally applicable 

 to No. IV, while No. VIII (very similar in composition to Nos. 

 II and IV) was taken to be a dike by Gorceix in his field 

 examination, though it is probable that if he had known its 

 composition a more detailed examination of this point would 

 have been made. 



In the same paper reference was made to a peculiar rock 

 from near the fall of the river Dattas which presents many 

 analogies with No. II but with more decided eruptive charac- 

 teristics. Both of these rocks were supposed to be allied to 

 No. 1, and when that on analysis proved to be chloritic rather 

 than sericitic, as was at first supposed, it was assumed that this 

 might also be the case with the two rocks in question. A 

 qualitative* analysis of the Dattas rock, however, shows that 



* As the rock is heavily charged with tourmaline, which is evidently an intro- 

 duced element, it was considerated that its original composition had been so 

 changed that the work of a quantitative analysis would not be compensated by 

 the results. 



