GEOLOGY. 



its hardness, forms an excellent material for street 

 paving and mending roads. 



Various precious minerals have been found in granite, 

 as the topaz, the beryl, the emerald, the garnet, &c. ; it 

 is also rich in veins of different metals. 



GREENSTONE. Greenstone is the chief of a family 

 called Trap rocks, from Trappa, the Swedish for a flight 

 of steps, they presenting some such appearance. 



All the Trap rocks, as Greenstone, Basalt, &c., as also 

 Porphyry, Amygdaloid, Lava, and Pumice, are supposed to 

 be of volcanic origin, and to have been once in a state of 

 fusion, so that, similar to lava, when it becomes solidified 

 it ends in a steep slope. Greenstone is composed of 

 felspar and hornblende, and, as may be imagined from 

 its name, is of a dark green or greyish colour. It is 

 found in England and Scotland, and in most parts of 

 the Continent, as well as in America. It is often used 

 for mending roads, and the late Mr. M'Adam preferred 

 it for that purpose to any other material. 



BASALT. This rock is of a dark green or blackish 

 colour, and is composed of hornblende with crystals of 

 augite. It is very generally of a columnar form, although 

 sometimes found in tabular masses. Edinburgh is built 

 on rocks of columnar basalt. The Giant's Causeway, in 

 the North of Ireland, and Fingal's Cave, in Staffa, are 

 composed of a magnificent range of basaltic columns. 

 The columnar structure is attributed to its having been 

 cooled under immense pressure, and contracting in a 

 manner similar to starch when first manufactured. 

 CLINKSTONE or PHONOLITE, GLADSTONE, and TRA- 

 CHYTE, are nearly allied to basalt, as they pass one into 

 the other. Clinkstone is so called from a peculiar ring- 

 ing noise it makes when struck ; it is of a greenish or 

 greyish colour, having a tendency to divide into slabs 

 and columns. Cliystone is an earthy stone resembling 

 indurated clay, and Trachyte* receives its name from its 

 roughness. The composition of trachyte is different 

 from that of basalt j trachyte being a felspathic rock, 

 while basalt is an augitic rock. 



PORPHYRY, AMYGDALOID, LAVA, and PUMICE, are 

 of igneous or volcanic origin. 



* T(u; rough. 



