SERPENTINE. TRAP. 355 



Potstone is a compact steatite. Rensselaerite is another 

 compact variety, (page 144,) found in Jefferson and St. Law- 

 rence counties, N. Y., and used for inkstands. 



SERPENTINE. 



This dark green rock is usually associated with talcose 

 rocks, and often also with granular limestones. It has been 

 described on page 145, where its uses are alluded to. It 

 often contains disseminated a foliated green variety of horn- 

 blende called diallage. A compound rock consisting of dial- 

 lage and feldspar, has been called diallage rock or eupholide 



TRAP. BASALT. 



Trap is a dark greenish or brownish black rock, heavy 

 and tough. Specific gravity 2*8 — 3*2. It has sometimes 

 a granular crystalline structure, and at other times it is very 

 compact without apparent grains. It is an intimate mixture 

 of feldspar and augite. It is often called dolerite. The 

 feldspar in this rock is usually the kind called labraborite. 

 (p. 178.) 



Amygdaloid, (from the Latin amygdalum, an almond,) is 

 a trap containing small almond-shaped cavities, which are 

 filled with some mineral : usually a zeolite, quartz or chlorite. 



Porphyritic trap is a trap containing, like porphyritic 

 granite, disseminated crystals of feldspar. 



Basalt resembles trap, but consists of augite, olivine and 

 feldspar. It varies in color from grayish to black. In the 

 lighter colored, which are sometimes denominated graystone, 

 feldspar predominates : and in the darker, iron, or a ferru- 

 ginous augite. The chrysolite (or olivine) it contains is 

 in small grains of a bottle-glass appearance. Magnetic or 

 titanic iron are also frequently present in the rock. When 

 feldspar crystals aie coarsely disseminated, it is called por. 

 phyritic basalt ; and when containing minerals in small 

 nodules, it is amygdaloidal basalt. Basalt is a common pro- 

 duct of volcanic action. 



WacJce or toadstone is an earthy basalt, or a sedimentary 

 rock of trap or basaltic material. 



Both trap and basalt occur in columnar forms, as at the 

 Giant's Causeway and other similar places. 



Trap and basalt are excellent materials for macadamizing 

 roads, on account of their toughness. Trap is also used for 

 buildings. It breaks into irregular angular blocks, and is 



