,586 



EEPORT OF NATIONAL MUSEUM, 1890. 



in presenting an amorphous base rather than being crystalline through- 

 out. These rocks are supposed to have an important bearing on the 

 origin of the diamond, the diamond bearing rocks of South Africa hav- 

 ing been found to be picrite porphyrite (Kimberlite, see specimen No. 

 73190 from the De Beers mine) cutting highly carbonaceous shales. 

 An examination of the Kentucky peridotite locality, where the rock 

 occurs under quite similar conditions, failed to show that similar results 

 had been there produced, a fact which is supposed to be due in part to 

 the small amount of carbonaceous matter in the surrounding shales. 

 The group is very limited, and is represented in the collection only 

 by samples from Elliott County, Kentucky (38603 and 38360); Pike 

 County, Arkansas, (72792) ; Syracuse, Onondaga County, New York 

 (35721 and 70556), and the De Beers diamond mine at Kimberly, South 

 Africa (73190), 



12. The limburgite and augitite group. 



Limburgite, a name given by Bosenbusch in 1872 to designate this 

 type of rocks as occurring at Limburg, on the Kaiserstuhl in theBhiue. 

 The name Augitite given, since augite is the essential constituent. 



These are small groups of eruptive rocks consisting essentially of the 

 mineral augite, with iron ores, and having a glassy base. Olivine is 

 present in the variety Limburgite. The common accessories are the 

 same as those of the basalts. Structurally the rocks so far as known 

 are never noncrystalline, but glassy and porphyritic. They are divided 

 into the olivine-bearing variety limburgite, and the olivine-free variety 

 augitite. The composition of (I), a limburgite from Bhenish Prussia, 

 and (II), an augitite from the Cape Verde Islands, as given by Both, 

 is as follows : 



Silica 



Alumina 



Iron sesquioxide 



Magnesia 



Lime 



Soda 



Potash 



Water 



II. 



Per cent. 



42.24 



8. 60 



7.45 



12.27 



11.76 



4.02 



1.08 



3.71 



Per cent. 

 41.83 

 18.00 

 1G. 11 

 4.98 

 11.83 

 4.70 

 2,47 

 0.91 



These rocks are of very limited distribution and at present quite un- 

 known within the limits of North America. The group is represented 

 in the collections as follows : 



Limburgiteis : Limburg ruin on the Kaiserstuhl, near Sasbach, Baden, 34718 and 

 36589; Hasenberg, Bohemia, 36588; near Xiririca, Prov. de Sao Paulo, Brazil, 

 69992; Nova Laraugerras, Prov. de Rio Janeiro, Brazil, 69991. 



Augitite: Province de Rio Janeiro, Brazil, 69995; Paschkapole, Bohemia, 36590. 



