ORIGIN AND DEVELOPMENT OF ORE DEPOSITS. 113 



convenient to consider original rock-masses ("deposits") under 

 the following heads, according to their origin : — 



1. Sedimentary. 4. .ZEolian. 



2. Organic. 5. Volcanic. 



3. Chemical. 6. Plutonic, 



(1). The sedimentary deposits will differ from each other 

 according to the nature of the materials of which they are 

 composed ; thus they may be argillaceous, calcareous, or siliceous ; 

 homogeneous or heterogeneous ; fine- grained or coarse-grained, 

 &c, while, according to the conditions under which they have 

 been deposited, they will be compact or laminated. In their 

 first stages it is probable that sedimentary rocks always appear 

 as mud, clay, sand, or gravel, and after alteration, as shale, 

 slate, sand-stone, conglomerate, marble, mica-schist, &c, &c. 



(2). The principal organic deposits are coral reefs and coal 

 beds, though there are others which consist largely of organic 

 accumulations and secretions. Coral reefs form immense compact 

 masses, sometimes roughly bedded, sometimes not. Coal is 

 usually distinctly bedded. Chalk may be regarded as an organic 

 rock, because its chief constituents have formed parts of organ- 

 isms, but in its mode of deposition it is essentially a sedimentary 

 rock. 



(3). Chemical deposits, such as travertin and siliceous 

 sinter, are, in the later geologic series, frequently connected with 

 mineral springs; but beds of rock-salt, gypsum, &c, are distinctly 

 lake deposits, and many of the Archaean rocks have probably 

 originated in a manner combining both kinds of phenomena. 



(4). iEolian deposits, or deposits of drifted sand, are of 

 considerable importance in some regions, as in the recent fossil- 

 iferous sand-stones of Guadaloupe and the sand-dunes of the 

 North Coast of Cornwall, some of which are consolidated into 

 real sand-stones of considerable hardness and durability. 



(5). "Volcanic rocks, such as obsidian and taehylite, trachyte 

 and basalt, which have been ejected in a fluid state from volcanoes, 

 are examples of this class. The first two are glassy, while the 

 third and fourth are largely or completely crystalline. Yesicular 

 structure is common in obsidian and trachyte, rare in basalt and 



