MANUAL OF NATURAL HISTORY. 543 



formed of quartz and hypersthene, with scattered 

 crystals of mica, it is HyperstTienic-granite ; when 

 it is mottled with chlorite, it is termed Serpentine ; 

 and when larger crystals of felspar are scattered 

 indiscriminately through it, Porphyry is the result. 



On account of its extreme hardness and dura- 

 bility, granite is chosen for the construction of im- 

 portant works, as bridges, docks, and lighthouses ; 

 the enduring monuments of ancient Egypt, the 

 Pyramids and Colossal Sphynx, are formed of 

 granite ; talc is often used for windows, and de- 

 composed felspar is employed in the manufacture of 

 china. Many Eastern works were formed of Sye- 

 nite, which is harder than granite; and columns, 

 monuments, and ancient antique works are fre- 

 quently of red, brown, green, or black porphyry. 

 The verde antico, or green porphyry of the ancients, 

 is composed of greenstone, with scattered crystals 

 of white and green felspar. Basalt is used in the 

 formation of sea-walls, and for road-stones ; some of 

 the Sphynxes and Lions sculptured by the Egypt- 

 ians, are of this substance. Trachyte, and felspathic 

 lavas, are sometimes employed as building materials, 

 as tufa, used by the Romans at Pompeii ; and Ser- 

 pentine or Ophiolite, a bisilicate of magnesia, has 

 been used extensively in the manufacture of idols, 

 columns, vases, and ornamental work. 



2. The Trap Rocks comprise the Basalts and 

 Porphyries, and commonly occur in the shape of 

 veins or dykes, as in the felspar porphyry rising- 

 through the granite which constitutes the base of 



