3S4 GEOLOGY, 



The volcanic materials are thought to have accumulated slowly and 

 to have been long exposed to the action of sea-water. The various 

 nodules and crystals in the clay are believed to be secondary products, 

 the materials for which were derived from the decomposition of the same 

 materials. Eolian dust may be a notable constituent of the red clay. 



Various specific products of chemical change may be briefly referred 

 to. The decomposition of certain mineral particles, such as feldspar, 

 gives rise to kaolin, and kaoHn is a very considerable constituent of most 

 of the clayey deposits of the ocean-bottom. The kaol'n'zation of feld- 

 spar may take place both on land and in the sea. Manganiferous de- 

 posits are widespread in the ocean-bottom, occurring both as coatings 

 on grains of mechanical sediments, shells, etc., and as concretions 

 ranging in sizes from minute particles to nodules an inch or more in 

 diameter. The concretions are sometimes approximately spheroidal, 

 but often botryoidal. These manganiferous nodules are believed to 

 have arisen from the decay of fragments of volcanic rocks. In their 

 decay, the manganese and iron are believed to have been first changed 

 to carbonates, and subsequently to oxides. After manganese oxide, 

 iron oxide and silica are by far the most abundant constituents, but 

 many other substances enter into their composition in minor quan- 

 tities. 



Another substance somewhat widely distributed in the sea-bed, though 

 by no means universal, is glauconite, a complex silicate of alumina, 

 iron, potassium, etc. Glauconite is, on the whole, most abundant 

 along the edges of the continental shelves, though it is by no means 

 universal in this position. It is not commonly found in deep water, 

 nor very near the shore, but approximately at the ''mud-hne." The 

 glauconite grains begin to form, as a rule, in tiny shells, chiefly the 

 shells of foraminifera. After filUng the shell, the shell itself may dis- 

 appear, while the glauconite goes on accumulating around the core 

 already formed, until the grain attains considerable size. Glauconite 

 is beheved to be an alteration product of certain sorts of mechanical 

 sediment, the change being effected under the influence of the decaying 

 organic matter in the shells.^ It does not occur where sedimentation 

 is rapid, and its formation appears to be favored by considerable changes 

 of temperature. Glauconite deposits occur on the land and are com- 



1 Challenger Report on Deep Sea Deposits, pp. 385-391. See also Jour, of Geol., 

 Vol. II, pp. 167-172. 



