130 ORVILLE A. DERBY 



the one presumed to be here represented. The two rocks are 

 often found united in the same rock-mass in such an intimate 

 manner that, given the almost perfect identity in aspect and 

 character, one is frequently inclined to doubt the evidence of 

 his senses even after unequivocal proofs of the existence of a 

 break have been detected. Similar cases of a complete blend- 

 ing of his massive and schistose types are graphically represented 

 in other sketches given by Helmreichen which can best be inter- 

 preted on the hypothesis of an unconformability. In the pres- 

 ent case, it seems to me that, admitting the accuracy of this 

 sketch, the appearance of a break is so evident that before 

 rejecting it most geologists would require much stronger evi- 

 dence than has yet been presented. 



A recent examination of a specimen with an inclosed diamond 

 in the collection of the National Museum at Rio de Janeiro 

 shows that the Grao Mogol rock contains both authigenic and 

 allothigenic elements to either of which groups, leaving out of 

 account considerations derived from other points, the diamond 

 might with equal plausibility be referred. The predominant 

 element quartz, which is presumably allothigenic, has by recrystal- 

 lization, secondary enlargement, or other process, taken on the 

 aspect of an authigenic element. The mica-like mineral (appa- 

 rently a clintonite) and the iron minerals, pyrite and martite 

 (magnetite ?) are certainly authigenic. Allothigenic elements 

 are represented (leaving out of account the pebbles which are 

 not well defined in the specimen in question) by distinctly rolled 

 zircons. Specimens of typical schistose itacolumite, which though 

 not from Grao Mogol may be taken as representing the supposed 

 lower series of that place, present the same mixture of authi- 

 genic and allothigenic elements (the latter represented by well 

 rolled zircons) and therefore the same evidence of clastic origin. 



As the case stands at present, the evidence from Grao Mogol 

 regarding the genesis of the diamond is inconclusive. The rock f 

 whether one or two series are represented, is a metamorphosed 

 clastic and no decisive evidence can be presented to place the 

 diamond in the class of either the authigenic or allothigenic ele- 



