330 PRINCIPLES OF STRATIGRAPHY 



substances which hll iij) fissures and oilier cavities, forming veins 

 and other types of (le])osits. Chief attention will here be given to 

 the deposits of carbonate of lime and magnesia, to gypsum and an- 

 hydrite, and to chloride of sodium and some other common salts. 



OCEANIC PRECIPITATES. 



Chemical DErosiTS of the Deep Sea. Purely hydrogenic 

 rocks, i. e., precipitates, are practically unknown in the open 

 ocean, but chemical deposits, in part alteration products, are 

 found. Here belong the manganese concretions, which are found 

 widely distributed over the floor of the abyssal portion of 

 the Pacific Ocean and elsewhere. They generally enclose foreign 

 objects, pieces of pumice, the teeth of sharks — often of extinct Ter- 

 tiary species — the ear-bones of whales, etc., or even fragments of 

 siliceous or calcareous sponges. The encrusting material is dis- 

 posed in concentric layers and consists of manganese oxide up to 

 63 per cent, and iron oxide up to 45 per cent., the iron in some cases 

 even exceeding the manganese. Clay and silica make up the re- 

 mainder, one or the other having been found to approach 50 per 

 cent, of the mass. Copper, nickel, lead, cobalt, molybdenum, and 

 traces of zinc, lithium and thallium have also been found in these 

 concretions. The source of the manganese has been thought to be 

 the sea water, in which it occurs as the bicarbonate, which, when 

 coming in contact with the oxygen of the air, is precipitated as the 

 peroxide. The small quantity of this salt found in the sea water 

 has, however, cast doubt on this explanation, and others have sotight 

 for the source of the manganese in the decomposition products of 

 basic volcanic rocks in the sea, or in su])marine volcanic eruptions 

 or in mineral springs. (See summary by Potonie-46:7(5(5.) An- 

 other alteration product is found in the crystals of Phillipsite, a 

 hydrous silicate of aluminium, calcium and potassium, found 

 abundantly in the deeper portions of the Paoific and Indian oceans. 

 They not infrequently occur in the form of crossed twins. These 

 are also regarded as due to precipitation of decomposition products 

 of submarine volcanic rocks, taken into solution in the water of the 

 deep sea. 



Glauconite, too, may be mentioned here. It accumulates in the 

 shallower portions of the sea, being more largely confined to the 

 littoral district, i. e., the continental shelf. Since this mineral is, 

 however, more properly to be regarded as an alteration product 

 of the finer marine sediments, its discussion is best deferred until 

 these sediments have been considered. (See Chapter XV.) 



