MICROSCOPIC TEXTURE OF LAVAS. 255 



appearance seen resembles the curvature and waved lines on some 

 kinds of marbled paper. Those rocks in which the fluidal structure 

 is best developed, are rich in broken crystals. 



Microliths. Among constituents found in the base of some lavas 

 are microlitks, which may be described as imperfectly formed needle- 

 shaped crystals. They may belong to many minerals, such as felspar, 

 hornblende, augite, mica, or apatite ; and if the microlith can be 

 identified, it is referred to its mineral species. Garnets have no 

 tendency to form microliths, and specular iron occurs in six-sided 

 plates. When the microlith is colourless it is a belonite, when it is 

 black and opaque it is a trichite. 



Opacite, Ferrite, and Viridite. Three other terms are used in 

 describing the base of rocks to designate substances which cannot be 

 certainly identified. First, opacite exists as black opaque grains, 

 and is found among the products of decomposition of minerals. It 

 may consist of oxides of iron, titanium, manganese, graphite, and 

 various earthy silicates. Secondly, ferrite is a rust-coloured material 

 of amorphous form, which cannot be identified, but is probably sesqui- 

 oxide of iron. Third, viridite is a fibrous or scaly-green transparent 

 substance, consisting of silicates of iron and magnesia. It results 

 from the decomposition of olivine, augite, and hornblende. The fibres 

 are referred to such a mineral as delessite, the scales to such a mineral 

 as chlorite. 



Propylite. 



In treating of the several kinds of volcanic rocks, 1 we have followed 

 the grouping of Bichthofen, chiefly because these rocks have been so 

 fully described in elaboration of his researches. And on this account 

 it has seemed desirable to give, in addition to a history of the 

 European volcanic rocks, a statement of the conditions under which 

 such rocks are found and their mineral variations in the western 

 volcanic region of the United States. 



Propylite. Eichthofen states that around Bisytritz, in Northern 

 Transylvania, propylite forms cones. It also occurs at Nagybanya 

 and Kapnik in Hungary. Massive eruptions of it are found on the 

 southern slopes of the Carpathians ; and it appears in every case to 

 have been ejected through fissures, since no traces of volcanoes of 

 propylite are known in Europe. The rock is always porphyritic, and 

 consists of a microcrystalline paste, of a dark-green or greenish-brown 

 colour, or red and grey. In the ground mass are scattered crystals of 

 white or light-green oligoclase, and dark-green fibrous hornblende. 

 The paste is formed of the same ingredients, with titaniferous iron ; 

 so that the colour of the rock is always green, and it closely resembles 

 oligoclase trachyte, and consists of the same ingredients as hornblende- 

 andesite. Some varieties contain rounded grains of quartz, and other 

 varieties hold crystals of augite. 



1 Rosenbusch's "Mikros. Physiographic der massige Gesteine " is a valuable in- 

 troduction to a knowledge of volcanic rocks, from which we have quoted many facts. 



