482 DR JOHN MURRAY AND MR ROBERT IRVINE ON THE 



two great classes, viz., those laid down in the deep water towards the central parts of 

 the great ocean basins, which are called pelagic deposits, and those laid down in shallow or 

 deep water in more or less close proximity to the land masses, which are called terrigenous 

 deposits. The terrigenous deposits * are estimated to cover about 28,000,000 square miles 

 of the sea-bed, or about one-seventh of the earth's surface. They extend from the shore 

 seawards to an average distance of 200 miles, and may be met with at a depth of over 

 2 or 3 miles. These terrigenous deposits are chiefly made up of materials derived from 

 the disintegration of the land masses. In shallow depths, where the bottom is swept by 

 waves and currents, these deposits consist chiefly of sands, gravels, and boulders ; but in 

 all hollow or cup-shaped basins within the 100-fathom line, and in all the greater depths 

 beyond 100 fathoms, the deposit is rarely disturbed by the motion of the water, and 

 generally consists of a fine plastic Blue Mud or Clay. The depth at which a fine mud 

 may form in the sea depends entirely on the depth of water and the extent of the basin ; 

 or, in other words, on the height and length of the waves.t In harbours it may be 

 deposited not deeper than 1 or 2 fathoms, while along the western coasts of Scotland 

 and Ireland, which are exposed to the waves of the wide and deep Atlantic, the true mud- 

 line may be situated at a depth of about 150 or 200 fathoms. 



The variety of terrigenous deposit to which the name of Blue Mud has been given 

 covers about 15,000,000 square miles of the sea-bed, and is chiefly found in estuaries, 

 harbours, enclosed seas, and along continental coasts where rivers pour their detrital 

 matter into the ocean. The Blue Muds collected by the "Challenger" ranged beyond 

 the 100-fathom line down to a depth of 2900 fathoms, the average depth being 1411 

 fathoms. The average percentage composition of the dried muds is as follows : — 



j Estimated to consist of f 1 '' "#' ' ! ' 



Other organisms, . . 321 



Carbonate of lime, ] , , t ^ Bottom-living Foraminifera, . . 1*75 



( the remains of • I ^., . 



12-48 



T? 'H ft r moval -> ( ^ e remains of siliceous organisms, 327 



r ', - v [ Estimated to consist of < Mineral particles, .... 22-48 



of carbonate ot lime, J | . . 



Fine washings, .... 61*77 



87-52 



In the Blue Muds of harbours, bays, estuaries, and generally in all positions within 

 the 100-fathom line, there is usually less carbonate of lime than in similar deposits in 

 deep water beyond 100 fathoms, and the mineral particles derived from the neighbouring 



* See Murray and Renard, Cludlcnger Report on Deep-Sea Deposits, London, 1891, p. 229. 

 + See Murray and Renard, op. cit., p. 185. 



