356 rocks. 



employed in this condition. For a Gothic building it is w« J 

 fitted, on account of an appearance of age which it has. 



PORPHYRY. CLINKSTONE. TRACHYTE. 



Porphyry consists mainly of compact feldspar, with dis- 

 geminated crystals of feldspar. Red or brownish-red and 

 green, are common colors ; but gray and black are met with. 

 The feldspar crystals are from a very small size to half or 

 three quarters of an inch in length, and have a much lightei 

 shade of color than the base, or are quite white. It breaks 

 with a smooth surface and conchoidal fracture. The specific 

 gravity and other characters of the rock are the same nearly 

 as for the mineral feldspar ; the hardness is usually a little 

 higher than in that mineral. 



Porphyry receives a fine polish, and has been used for 

 columns, vases, mortars, and other purposes. Green por- 

 phyry is the oriental verd antique of the ancients, and was 

 held in high esteem. The red porphyry of Egypt is also a 

 beautiful rock. It has a clear brownish red color, and is 

 sprinkled with small spots of white feldspar. 



Clinkstone or Phonolite is a grayish-blue rock, consisting, 

 like porphyry, mainly of feldspar. It passes into gray basalt, 

 and is distinguished by its less specific gravity. It ring? 

 like iron when struck with a hammer, and hence its name. 

 Trachyte is another feldspathic rock, distinguished by 

 breaking with a rough surface, and showing less compact- 

 ness than clinkstone. It sometimes contains crystals of 

 hornblende, mica, or some glassy feldspar mineral. It occurs 

 in volcanic regions. 



LAVA. OBSIDIAN.— PUMICE. 



The term lava is applied to any rock material which has 

 flowed in igneous fusion from a volcano. Basalt is one kind 

 of lava ; and when containg cellules, it is called basaltic 

 lava. Trachyte is also a lava. There are thus both feld- 

 spathic and basaltic lavas. The feldspathic are light colored, 

 and of low specific gravity, (not exceeding 2*8) ; the basaltic 

 vary from grayish-blue to black, and are above 2*8 in specific 

 gravity. The general term basaltic sometimes includes 

 'doleritic lava, which is closely allied. Chrysolite is present 

 in basaltic lavas ; and such lavas are not unfrequently por 

 phyritic, or contain disseminated crystals of feldspar. 



