212 



THE GUIDE TO NATURE. 



These are masses of rock that have 

 been detached and rolled around by 

 nature, generally by brooks or rivers, 



l "<* 



FIG. 2. INTERIOR OF A SELENITE CAVE. 



until their rough edges have been 

 rounded off, when they are called peb- 

 bles if small and boulders if large. The 

 picture given herewith is a boulder 

 composed of two minerals, giving it a 

 quite peculiar appearance. The white 

 streaks are veins of quartz and the 

 dark parts are composed of sandstone. 

 The white pebbles so often gathered 

 from beaches are simply clear quartz. 

 At some of the beaches visited by tour- 

 ists the demand for these clear pebbles 

 is so great that they are made artifi- 

 cially by putting some pieces of clear 



quartz into a receptacle along with 

 some sand and this is then closed up 

 and revolved until the pieces become 

 rounded, when they are sold to the 

 tourists as natural pebbles and are 

 often cut and mounted, as watch 

 charms or other ornaments. 



THE BOULDER COMPOSED OF QUARTZ AND 

 SANDSTONE. 



Bronx Tourmalines 



BY EDWIN W. HUMPHREYS, NEW YORK 

 CITY 



Tourmaline is one of the most beauti- 

 ful and attractive as wefl as one of the 

 commonest minerals found in the Bor- 

 ough of the Bronx, Xew York City. 

 While it has not the beauty of color 

 or transparency that the varieties found 

 in Maine and California have, it does 

 have that perfection of form which 

 gives so much interest to good crys- 

 tals of even the commonest of minerals. 

 Instead of being brilliantly colored red, 

 green or yellow as are tourmalines of 

 many other localities, the Bronx speci- 

 mens are modestly colored, being coal 

 or pitch black and brown. At some lo- 

 calities, moreover, there are found tour- 

 malines whose extremities are of dif- 

 ferent colors, one end, for example, 

 being red and the other green or some 

 other color. This phenomena ao'oarent- 

 lv does not occur among the Bronx speci- 

 mens, which in so far as I have seen 

 them, always of a uniform color. 

 Neither the black nor the brown is 

 transparent ; the nearest approach to it 

 being in the brown variety which is 

 slightly translucent. Crystals of the lat- 

 ter are sometimes curved. 



Specimens of the black tourmaline are 

 often of great beauty. I remember find- 

 ing one. in particular, in which a mass of 

 the pinkish or flesh-colored feldspar so 

 common in the pegmatite dikes formed 

 the background for a fine, slender, coal 

 black crystal. These black crystals vary 

 greatly in size, some being hairlike and 

 a fraction of an inch in length, while 

 others are several inches across and 

 many inches long. Frequently the ter- 

 mination planes are very well developed. 

 Occasionally, the crystals are cracked 

 across and the interstices filled with 

 quartz or mica, at other times plates of 

 mica are apparently included within the 

 crystal itself. This condition can only be 

 found, however, by breaking the crys- 



