BUILDING AND ORNAMENTAL STONES. 437 



five years other than a slight and in no way objectionable darkening of 

 color. Neither stone has been used as yet for other than paving pur- 

 poses and bridge abutments, though they are apparently well adapted 

 to all kinds of work for which their color and hardness qualify them. 



(2) GABBRO. 



The rock gabbro differs from diabase mainly in containing the foliated 

 pyroxene diallage in place of augite. It is not at present quarried to 

 any extent in this country, though for no apparent reason other than 

 that it is difficult to work. 



Yery extensive outcrops of a dark gray, almost black gabbro of 

 medium fineness of texture occur in the immediate vicinity of Balti- 

 more, Md., but which have been quarried only for purposes of rough 

 construction close at hand. The rock is popularly known as ''nigger- 

 head" owing to its hardness, dark color, and its occurrence in rounded 

 bowlders on the surface.* 



At Rice's Point, near Duluth, Minn., there occurs an inexhaustible 

 supply of a coarse gabbro, which has been studied and described by 

 Professor Winchell. t The feldspar of the rock, which is labradorite, 

 according to the authority quoted, sometimes prevails as at Beaver Bay, 

 in crystals one-half to three-fourths of an inch across, and to the almost 

 entire exclusion of other constituents. In this form the rock varies from 

 lavender blue or bluish gray to light green, and acquires a beautiful 

 surface and polish, and is considered as constituting a valuable material 

 for ornamental slabs and columns. The typical gabbro of the region is 

 of a dark blue-gray color, and u has been employed in a few buildings 

 at Duluth, both in cut trimmings and for rough wals." It has also 

 been used for monuments and for bases, to which it is especially adapted, 

 being cut under the chisel and polished more easily than any of the 

 crystalline rocks that contain quartz. The stone is known popularly 

 as u Duluth granite." The same kind of rock occurs at Taylor's Falls, 

 but is little used, though favorably situated for quarrying and trans- 

 porting. 



A rock closely allied to the gabbros and diabases is the so-called 

 norite, which consists essentially of the minerals hypersthene and a 

 plagioclase feldspar. The only rocks of this Dature now regularly 

 quarried are at Keeseville, K Y., and Yergennes, Yt. The first is 

 known commercially as "Au Sable granite," and the second as "Labra- 

 dorite granite." Both are coarse-grained, dark-gray rocks, much resem- 

 bling the darker varieties of the Quincy granites, from which, however, 

 they differ radically in mineral composition. They take a high lustrous 

 polish, frequently show a beautiful bright bluish iridescence, and are 



* This is the rock the interesting petrographical features of which have lately been 

 made known by Dr. Williams, of Johns Hopkins University. See Bull. U. S. Geol. 

 Survey, No. 28. 



tGeoL of Minn., Vol. i, pp. 148-9. 



