pratt.] MODES OF OCCUERENCE. 19 
the alumina is used in uniting with these oxides and with silica to 
form feldspar. 
5. There is a strong tendency for the alumina to unite with the 
alkali and the alkali-earth oxides to form double silicates like feld- 
spars, whether such silicates form the chief minerals of the resulting 
rock (»!• are present in relatively small amount. There is, however, 
Little tendencj^ for the alumina to unite with magnesia to form double 
silicates when the magma is a magnesium silicate. 
CORUNDUM IN BIOTITE, CONTACT ON SAXONITE. 
At the asbestos quarry near Pelham, Mass., there is a Large lens of 
the igneous saxonite variet}^ of the peridotite rocks penetrating the 
acid gneiss of the country. The saxonite is very much altered to a 
depth of 3 to 12 feet, when the hard nearly fresh rock is encountered, 
which is of a dull-black color and is made up of grains of olivine and 
the orthorhombic pyroxene, bronzite. The black color of Hie rock is 
due to disseminated pari icles of chromite and magnel ite. Part of the 
magnetite may be due to the alteration of t he grains of olivine, similar 
to that observed in the dunites of North Carolina, where, a1 1 he begin- 
ning of its alteration, there is a deposition of magnetite in tine grains, 
which forms a network of black lines often outlining the grains of 
olivine. 
Professor Emerson has made a petrographical examination of this 
rock and describes it as follows: 1 
This is a very fresh mixture of olivine and enstatite, both dusted through with 
black ore. largely chromite. It is a dull-black rock of very great toughness. The 
olivine grains have often many crystalline faces. The enstatite is in rare, small 
plates, with parallel sides and irregular ends, and with a fine wavy lamination, 
which is often marked by lines of black ore. generally concentrated in some part 
of the plate, especially the center. Although nearly colorless or pale bronzy in 
common light, it has marked pleochroism. It is plainly rhombic, and grades into 
the asbestiform decomposition product in veins running through the section. 
None of the anthophyllite that is so abundant in the decomposed 
portion of the saxonite were observed in the fresh rock. 
From the fresh rock to the surface and the contact with the gneiss 
the saxonite is more or less completely altered, and penetrating through 
this there is a network of veins of anthophyllite, which are more or 
less asbestiform. These veins vary in width from very thin seams to 
8 inches, with some that are very much wider, from which fibrous 
masses have been obtained 20 to :><> inches long. It is these veins of 
fibrous anthophyllite that constitute the asbestos quarry of Pelham. 
The saxonite is separated from the gneiss by a band of bronze- 
colored biotite, usually 4 to 8 inches thick, but in places reaching a 
thickness of 1 feet. In this wider portion there are nodules or im- 
perfect crystals of corundum of a grayish color mottled with blue. 
1 Moil. U. S. Ucol. Survey Vol. XXIX, p. 5^. 
