,8,.. DEEP-SEA DEPOSITS. 21 



embedded in the glass are those of basic igneous rocks — oUvine, 

 augite, and plagioclase. The fragments of basic glass are very 

 commonly associated with manganese nodules, and often form the 

 nuclei round which deposition has taken place. In some localities 

 actual deposits of palagonitic tufa, with the different lapilli in their 

 original relative positions, have been brought to the surface enclosed 

 in the larger and somewhat tabular concretions. The change from 

 basic glass to palagonite is shown by analysis to be accompanied by 

 hydration, removal of lime and magnesia, oxidation of the ferrous 

 iron, and addition of alkalies. 



Non-volcanic minerals and rocks are, as a rule, absent from the 

 pelagic deposits. The most characteristic minerals are glaucophane, 

 white mica, sericite, tourmaline, zircon, and microcline. Most of the 

 quartz is also of non-volcanic origin. It is in the terrigenous deposits 

 of the continental slope that the above minerals are found. 



When the particles which can be extracted from a deep-sea 

 deposit by a magnet are examined under the microscope, a few 

 black and brown spherules may not unfrequently be observed 

 amongst them. These are supposed to be of extra-terrestrial origin. 

 The black spherules rarely exceed •2 mm. in diameter. They have an 

 external crust of magnetic oxide of iron, and usually an internal 

 metallic nucleus. They would be perfectly spherical in form if it 

 were not for a slight depression or cupule which is almost invariably 

 present. The metallic nucleus either consists of iron or of iron 

 alloyed with cobalt and nickel. The brown spherules have a mean 

 diameter of '5 mm. and a peculiar radial, excentric, lamellar structure 

 known only in the chondres of meteorites. At first they were 

 believed to be formed of bronzite, the mineral of which the chondres 

 are composed, but more detailed examination has led the authors to 

 the conclusion that the mineral in this case is a monoclinic pyroxene. 

 Blackish-brown inclusions, similar to those occurring in hypers- 

 thene, are present in these peculiar bodies, and it is to these 

 inclusions that their magnetic properties are due. 



The black and brown spherules were found in the greatest 

 abundance in the red clays of the Central and Southern Pacific. 

 Twenty or thirty black, and five or six brown spherules may usually 

 be obtained from a quart of the clay. Manganese nodules, sharks' 

 teeth, and the ear bones of whales abound in the same localities. 

 In the pelagic deposits of the Central Pacific, other than red clay, 

 magnetic spherules are far less abundant, but a careful search 

 through a large quantity will usually result in the discovery of one 

 or two. These facts clearly point to the conclusion that the spherules 

 are most abundant where the rate of accumulation is slowest. 



The chemical deposits forming in situ on the floor of the ocean 

 are considered under the following heads : — (i) Clay, (2) Manganese 

 nodules, (3) Glauconite, (4) Phosphate nodules, (5) Zeolites. The 

 argillaceous matter of the red clay and other pelagic deposits is 



