HANDBOOK FOR THE DEPARTMENT OF GEOLOGY. 561 



Colors. — The prevailing colors are gray to nearly black ; sometimes 

 greenish through decomposition. 



Classification.— The rocks of this group are divided into (1) the true 

 gabbros — that is. plagioclase diallage rocks — and (2) norites or plagio- 

 clase bronzite and hypersthene rocks. Both varieties are further subdi- 

 vided according to the presence or absence of olivine. We then 

 have : 



True gabbro = Plagioclase -f- diallage. 

 Olivine gabbro = Plagioclase + diallage and olivine. 

 Norite ^= Plagioclase -f- hypersthene or bronzite. 



Olivine norte = Plagioclase -f- hypersthene and olivine. 



Nearly all gabbros contain more or less rhombic pyroxene, and hence 

 pass by gradual transitions into the norites. (Illustrated in specimens 

 38429, 39016 and 30754.) Through a diminution in the proportion of 

 feldspar they pass into the peridotites, and a like diminution in the pro- 

 portion of pyroxene gives rise to the so-called forellenstein. (Speci- 

 mens 30522 and 30523, from Volpersdorf, Silesia.) Uyperite is the name 

 given by Tornebohm to a rock intermediate between normal gabbro 

 and norite. (Specimen 70410, from Norway.) 



The following are the principal localities and varieties now repre- 

 sented : 



Gabbro: Gilford and Waterville, New Hampshire, 37048, 37047, and 27856; Beaver 

 Bay, Lake County, Minnesota, 26571 and 26632 ; Washingtonville, Orange 

 County, New York, 36129 ; Coverack, Lizard District, England (the diallage rock 

 of De la Beche), 39003; Banile, Norway, 70410; Volpersdorf, Silesia, 36510 to 

 36514 inclusive; Schmalenberg, Prussia, 36515 and 36516; Monte Ferrato, near 

 Prato, Italy, 73056; near Porretta, Province of Bologne, Italy, 36517, 36518, and 

 73059; Monte Cavaloro, Bologne, Italy (oligoklasit), 36519 and 73058; Kijor, 

 South Russia, 38,809. 



Olivine gabbro: Pigeon Point, Minnesota, 70611; Pikesvillo, Maryland, 69556; 

 Gwynn's Falls, Maryland, 36753 ; Volpersdorf, Silesia, 36520 to 36523, inclusive ; 

 Elfdalen, Sweden, 36524; Snarum, Norway. 36525 ; Bamle, Norway, 36526; An- 

 son, North Carolina, 39016; Ilchester, Maryland, 69552; near Baltimore, Mary- 

 land, 36754 aud 36755 ; the last a gabbro diorite resulting from the molecular 

 alteration of the gabbro No. 36754, both samples being broken from the same 

 rock mass. 



Norite : Keeseville, Essex County, New York, 38744, 38748, and 29346, the last with 

 garnets ; Port Kent, New York (hyperito), 36125 ; Kragero, Norway (hyperite), 

 70416; Ebersdorf, Saxony (hypersthenite), 34680; Radanthal, Harz Mountains, 

 Germany (hypersthenite aud bronzite anorthite rock), 36527 and 73045 ; Barro do 

 Pratorio, Rio Tubaras, Province of Sta. Catharina, Brazil (olivine norite), 69676; 

 Baste Harz (bastite), 36528. 



Hypersthene gabbro : West side of Ramsas Mountain, about 35 kilos southeast of 

 Christiana, Norway. (Large mass on special pedestal, 38429). This rock is a 

 peculiar variety of gabbro and has been called "Potato Rock" on account of its 

 fancied resemblance to a mass of potatoes imbedded in a matrix. It consists, ac- 

 cording to Prof. Meinich, of " kugels,' or concretionary nodules of greenish-brown 

 labradorite, greenish oligoclase, scales of brown and green mica, and abundant 

 magnetite. It occurs associated with the nickel ores of Smalenene. 



H. Mis. 129, pt. 2 30 



