ON THE GENESIS OF THE DIAMOND 1 4 1 



present in abnormal abundance, contact metamorphism is 

 strongly suggested. The residue of the white clay, on the other 

 hand, gives no indication as to its origin, since the only char- 

 acteristic accessory found in the small quantity available for 

 washing is rutile, which is so widespread and varied in its mode 

 of occurrence and association as to be indeterminative. One of 

 the washings from the colored clay gave two types of zircons, 

 the usual round, much worn reddish ones, and less worn whit- 

 ish elongated prisms. The latter resemble those already men- 

 tioned as occurring (with a fresher appearance, however,) in a 

 miner's concentrate, and still more closely those of a partially 

 decomposed rock from the Sopa mine in the neighborhood 

 (where lithomarge also occurs, but is not known to be dia- 

 mantiferous), which strongly resembles the European " porphy- 

 roid," and is either metamorphosed arkose or porphyry, probably 

 the former. 



This white clay, in the character of its material and of its 

 contacts, and in the lack of characteristic clastic elements, is 

 strongly suggestive of the so-called pegmatite veins that are of 

 frequent occurrence in similar formations and under similar con- 

 ditions. The quartzose character of some of the veins, or parts 

 of veins, is not inconsistent with this hypothesis, as the intimate 

 relations and interdependence of quartz and pegmatite veins are 

 well known. The indications furnished by this body are there- 

 fore in accord with those of No. I — that is to say, that the vein 

 matter was probably originally pegmatitic, and that it was 

 accompanied by phenomena of contact metamorphism. 



So far as can be made out from the observations thus far 

 made on material the most unsatisfactory that can be imagined 

 (foliated and highly modified by dynamometamorphism and 

 afterwards totally decomposed so as to present, in its present 

 state, one of the most intricate problems of mud geology), the 

 most plausible hypothesis as regards the various clays of the 

 Sao Joao da Chapada mine is that they represent an original 

 group of phyllites of varied character, but principally, if not 

 exclusively, of clastic origin, threaded with veins of pegmatite. 



