420 NATURAL HISTORY. 



or foatn-like stone, produced by volcanoes, and, from its 

 light, spongy structure, swims on the surface of 'water. 



Pozzuolana is a kind of tufa (a sand rock, consisting 

 of volcanic material, either cinders or the comminuted 

 lava) found in the vicinity of Rome, and is much valued, 

 as, mixed with chalk or lima, it hardens in water. It is 

 of great importance in the manufacture of molds or plaster 

 images. Trass, used for the same purposes, is but. a 

 variety of the foregoing. Both form a hydraulic cement. 



Trap Basalt Basalts which resemble Trappean of 

 peculiar formation, their columnar structure being a 

 remarkable characteristic. They usually stand nearly 

 perpendicular, their structure divided into regular prisms, 

 with sides varying from three to eight, but mostly five or 

 six. This irregularity appears to have been caused by 

 objects or other substances found in the path of the fluid 

 basalt, which is compact lava, as the basaltic pillars ex- 

 hibit more regularity at their bases. The texture of 

 basalt is fine-grained and compact, and often contains 

 fragments of other minerals imbedded in it, such as feld- 

 spar, quartz, mica leucite, and oxide of iron, which were 

 broken or scaled off from their original rocks by the vol- 

 canic eruption, and borne along in the stream of fluid or 

 semifluid lava. Wherever basalts are found in crevices 

 or fissures of other rock formations, they are observed to 

 retain their prismatic and pillar-like form, which is sup- 

 posed to be caused by some peculiar atmospheric influence 

 during their cooling. They also exhibit hollow cavities 

 or vesicles, apparently formed by bubbles of air during 

 their fusion. The Giant's Causeway, in the North of 

 Ireland, is composed of basaltic columns. 



