1887.] 415 rShaler. 



of incoherent material which will be shaken into the water by an 

 earthquake disturbance. Even where there is no other source of 

 sediment than the organic remains themselves, the bottom is cov- 

 ered with a sheet of ooze. By far the greater part of the organic 

 remains, deposited on the sea floor, fall into the condition of mud 

 or are brought into that state by the action of creatures, such as 

 our boring worms, which obtain their subsistence from the hard 

 parts of various dead creatures. A very slight annual contribution 

 of such material maintains a considerable depth of incoherent mat- 

 ter which is only gradually converted into deposits of such solid- 

 ity as will resist the action of an earthquake shock and not be 

 shaken up into the water. 



Observations on our crinoidal limestones serve to confirm the 

 foregoing hypothesis. It is well known that where strata are mainly 

 composed of the remains of crinoids the beds are usually very 

 much thicker than those which are formed of mollusks. The rea- 

 son for this is plain. The crinoidal species are in most cases sup- 

 ported on flexible stems of considerable height. In many cases 

 the sea floor, particularly in the sub-carboniferous period, appears 

 to have been covered by vast groves or thickets of these animals, 

 the stems standing almost as close as those of wheat upon a field. 

 When subjected to an earthquake shock it is not likely that these 

 creatures would be seriously damaged. Their elastic stems would 

 in a measure protect them from the direct blow and as their mouths 

 were elevated several feet above the plane of the bottom, it is not 

 likely that the invasion of mud would have any considerable influ- 

 ence upon them. At various points we may observe where ap- 

 parently an invasion of mud has somewhat affected the various 

 lowly forms of life which dwelt upon the bottom in the spaces be- 

 tween the crinoid columns. Still the principal element of the sea 

 floor life, the crinoids themselves, have maintained their existence 

 and prevented the formation of a layer of pure clay where otherwise 

 such deposits would have been made. 



Again where the limestone deposit is made by the remains of 

 floating animals, and is dropped to the bottom as is probably the 

 case with a part, at least, of our chalk deposits and also with the 

 deposit now forming on what has been termed the telegraphic pla- 

 teau of the North Atlantic, the accumulation is not arrested by any 

 disturbance affecting the sea floor. It thus may come about that 

 very massive limestone strata are slowly deposited. 



