liUILDlNG AND ()i:.N AMHNTAL STONKS. 437 



five years other tluin a> slight and in no way objeciioiiable (hnkeiiiiij;- of 

 coU)r. Neither stoiic has been used as yet for other than paving pur- 

 l)oses 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 gubbro ditfers from diabase mainly iu containiug the foliated 

 I)yroxeue diallage iu place of augite. It is not at present quarried to 

 any extent iu this countr}', though for no apparent reason other than 

 that it is difficult to work. 



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

 medium fiueuess 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 ii coarse gabbro, which has been studied and described by 

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

 acconling 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 vahuible material 

 for ornamental slabs and columns. The typical gabbro of t'lie region is 

 of a dark blue-gray color, and "has been employed in a tew buildings 

 at Duluth, both in cut trimmings and for rough walfts," 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 auy of the 

 crystalline rocks that contain quartz. The stone is known popularly 

 as " 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 nature now regularly 

 quarried are at Keeseville, N. Y,, and Vergennes, Vt. The first is 

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

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

 bling the darker varieties of the (^uincy 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 tli(3 rock the iiitcrcwtinj; ])()UujL;raphit;;il f<'a(urc8 of which have hitoly been 

 made known by ])r. Williams, of Johns Hopkins University. See Bull. U. tt. Geol. 

 yiirvey, No. 28. 



tGcoI. of JNliuu., Vol. I, pp. 148-9. 



