DOMAIN -II. SILICEOUS. 



only short and irregular veins, terminating in 

 solid masses; while in gneiss the veins are uni- 

 form, and regularly divide the whole, as in slate, 

 or in other substances properly schistose Hence 

 gneiss has also been called schistose granite by 

 the French, and other writers. 



In gneiss the mica is generally more abund^ 

 ant, as dividing the substance into regular plates. 

 Sometimes the place of mica is supplied by 

 siderite, which, as already explained with regard 

 to granite, cannot be regarded as altering the 

 denomination, but is only a proof of greater an- 

 tiquity. The siderite is also sometimes inter- 

 spersed in thick layers, or even beds. One of 

 the most interesting kinds of gneiss, is that with 

 Red. red felspar, sometimes of a wavy or undulated 

 structure, and which is also considered by some 

 as the most ancien . This, like other schistose 



contorted, substances, is found contorted, or convoluted, in 

 fantastic forms; by some regarded as originating 

 from internal expansion or disturbance; while 

 others consider it as the mere effect of a parti- 

 cular crystallisation. 



Primary. Geologists in general have considered granite 

 as the oldest substance, the fundamental rock 

 which supports all the others: the Huttonians 

 however regarding it, on the contrary, as the 

 newest substance, which, being elevated by ex- 



