SOLID VOLCANIC PRODUCTS. 121 



Hornblende (from the German), a kind of dark or black variety of mine 

 al, belonging to the same group as tre'molite, acti'nolite, asbe'stus, &,c. 



Mi'cu (from the Latin, mico, I shine), is a mineral generally found in 

 ihin, elastic laminae, soft, smooth, and of various colours and degrees of 

 transparency. It is one of the constituents of granite and its associate 

 rocks. 



40. 2d. Obsi'dian (from the Greek, ops/5, view, or after Obsi- 

 dius, who first found it in Ethiopia"), is a homogeneous, vitreous 

 substance of various colours. By me ancients it was used in 

 place of glass, and is also called volcanic glass. It consists of si'li- 

 ca, alu'mina, with a little potash and oxide of iron. 



This substance is produced abundantly in the islands of Lipari 

 and Teneriffe, the volcans of the Andes, and wherever volcanic 

 apertures open in tra'chyte. 



41. 3d. Compact lava. A substance with a compact base of a 

 deep colour, most frequently formed of la bradorite, containing crys- 

 tals of the same substance, or of the feldspa'thic group in general, 

 which in the mass presents a more or less distinct porphyritic struc- 

 ture. Crystals of py'roxene, of am phibole, black mica and peri- 

 dote are also occasionally found. 



La'bradorile L ibrador spar. A beautiful variety of opalescent feldspar 

 from the coast of Labrador : it exhibits brilliant and mutable tints of blue, 

 red, green and yellow, and is susceptible of a good polish. It is cut into 

 small slabs, and employed in ornamental jewelry. It is a si'licate of alu'. 

 mina, lime, and soda, with truces of oxide of iron. 



1'y'rozene (from the Greek, pur, fire, and zenos, stranger). The augite, 

 supposed to have pre-existed in the volcanic minerals containing it, and not 

 to have been formed by fire. 



Am'phibole (from the Greek, amphibolos, equivocal). A name applied by 

 some mineralogists to hornblende, because it may be mistaken for augite. 



Peridot e, or Chrysolite (from the Greek, chrusos, gold, and lithos, stone), 

 from its colour. The topaz of the ancients. 



These substances constitute the centre of thick currents, the in- 

 ferior part of the mass formed in excavations or hollows ; they are 

 often divided into prismatic columns. 



42. 4th. Porous, or scoria'ceous lava. A substance of the 

 same nature as the preceding, but rarely having crystals embedded 

 in it, and its structure is porous, or cellular. These lavas consti- 

 tute the upper parts of thick layers, and envelope lava currents 

 and streams which rest on the surface of the ground. 



43. 5th. Pouzzolani, volcanic tufa. Masses of small scoria'- 

 ceous fragments, or rupilli, accumulated around volcans, or earthy 

 substances, which contain them in greater or less quantity. Pu- 

 mice-tufas are formed of fragments of pumice, and trdchytic con- 

 glomerates of fragments of tra'chyte, united by crystalline or earthy 

 cement. 



40. What is obsi'dian ? 41. What s compact lava 7 



42. What is scoria'ceous lava? 43. What is volcanic tufa? 



11 



