II. 

 Murray's Mu(:M>ine. 



THE quotations from Professor Herdman, contained in an editorial 

 note in the October number of Natural Science, do not convey 

 a quite correct idea of what Dr. Murray has written with regard to 

 the position of the mud-line off continental shores. As the 

 "Challenger " volumes are not very readily available, the following 

 extracts bearing on the subject will, I feel sure, be interesting to 

 many of your readers : — 



" Notwithstanding some exceptions, due to special conditions, as, 

 for instance, on deep ridges between oceanic islands, where gravelly 

 deposits are found, or in bays, fjords, and enclosed seas, where mud 

 is met with in shallow water, it may be said that, along all coasts 

 situated in or fronting the great oceans, loo fathoms is the average 

 depth at which fine mud or ooze commences to form. At about this 

 depth the deposits on the whole assume a greater uniformity of 

 composition and grain, and the signs of mechanical action tend to 

 diminish or completely disappear. The greater the extent and depth 

 of the ocean, the greater the depth to which wave-movement extends, 

 and consequently the greater is the depth at which the mud-line is 

 formed around the coasts, but the average depth of this mud-line may 

 be taken as approximately about loo fathoms." . . . . " We have 

 seen that while the depth at which these muds form in enclosed seas 

 or estuaries may be only a few fathoms, yet along all the continental 

 shores facing the great ocean basins the average depth of the mud-line 

 may be taken at loo fathoms." (" Deep-Sea Deposits," pp. 185, 

 228, 229.) 



" The 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 loo-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 m 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. In 

 harbours it may be deposited not deeper than i or 2 fathoms. 



