560 REPORT OF NATIONAL MUSEUM, 1890. 



Quartz diorite : Schwarzenberg, Saxony, 36362 ; Monte Tonale, in the Tyrol (tonalite), 

 36361; Mount BLauc, 361 ( J ; Scbeinenitz, Hungary, 26371 ; Neuntestein in the 

 Vosges, 36363 ; Weinheim, Baden, 70207. 



Augile diorite : Scheuinitz, Hungary, 36373 and 36374 ; Cziclova Band, Hungary (with 

 quartz), 70208. 



Of the dioritii), lainprophyrs, are exhibited: Camptonile, Campton Falls, New Hamp- 

 shire, 29424 and 29425 ; Livermore Falls, New Hampshire, 29452 ; North Lisbon, 

 New Hampshire, 38042 ; Dixville Notch, New Hampshire, 29422; Forest of Dean 

 Iron mines, Orauge County, New York, 39399. 



Kersantite : Kersautou, Finistere, France, 36356 ; Nassau, Germany, 36357 and 36360 ; 

 Michselstein, Hartz Mountains, Germany, 70206 ; Wilischthal in the Erz-Gebirge, 

 Saxony, 36358 ; Markish in the Vosges Mountains, 36359 ; Franklin Furnace, New 

 Jersey, 39039. 



5. The gabbros. 



Gabbro, an old Italian name originally applied to serpentinous rocks 

 containing diallage. 



Mineral composition. — The gabbros consist essentially of a basic soda 

 lime feldspar, either labradorite, bytownite or anorthite, and dillage or 

 a closely related monoclinic pyroxene, a rhombic pyroxene (enstatite or 

 hypersthene), and more rarely olivine. Apatite and the iron ores are 

 almost universally present and often picotite, chromite, pyrrhotite, more 

 rarely common pyrites, and a green spinell. Secondary brown mica and 

 hornblende are common. Quartz occurs but rarely. 



Chemical composition. — The average of seven analyses as given by 

 Professor Zirkel is as follows : Silica 50.17, alumina 16.39, oxide of 

 iron 11.91, lime 9.49, magnesia 6.06, potash .93, soda 2.67, ignition 1.84 

 per cent. Specific gravity, 2.85 to 3.10. 



Structure. — The gabbro structure is quite variable. Like the other 

 plutonic rocks mentioned they are crystalline granular, the essential 

 constituents rarely showing perfect crystal outlines. As a rule the 

 pyroxenic constituent occurs in broad and very irregularly outlined 

 plates, filling the interstices of the feldspars which are themselves in 

 short and stout forms quite at variance with the elongated lath-shaped 

 forms seen in diabases. This rule is, however, in some cases reversed, 

 and the feldspars occur in broad irregular forms surrounding the more 

 perfectly formed pyroxenes. (See very coarse gabbro from Monte Fer- 

 rato, near Prato, Italy, No. 73056.) Transitions into diabase structure 

 are not uncommon. In rare instances the pyroxenic constituents occur 

 in concretionary aggregates or kugels as in the peculiar gabbro from 

 •Smaalaneue in Norway (see large specimen No. 38429). Through a 

 molecular change of the pyroxenic constituteut the gabbros pass into 

 diorites as do also the diabases. A beautiful illustration of this is given 

 in the gabbro about Baltimore, Maryland, described by Dr. George H. 

 Williams, and represented in the collections by specimens 36754 and 

 36755. Both are from the same rock mass, but one is a typical gabbro 

 while the other would be classed as a diorite if judged by its mineral 

 composition alone. 



