IIO THE HABITAT OF THE EURYPTERIDA 



forming the Jurassic Plattenkalke of Solnhofen, is shown by the utter 

 absence in this horizon or vicinity of reefs which could furnish such 

 deposits, and again by the presence in the composition of the silica 

 and alumina. In the Bertie the silica and the alumina is inti- 

 mately mixed with the lime, as is shown by the relative constancy in 

 composition and character of specimens from different parts of the 

 formation. In the Plattenkalke of the Solnhofen, on the other hand, 

 where the siliceous material represents the impure dust blown from 

 the land, it is found in clayey layers (Faulen) between the thin 

 bedded (Quicksteine) and thick bedded (Flinze) limestones, and not 

 in intimate "mixture with the other constituents, as is the case in the 

 Bertie (293, 144, 209). 



(2') The only remaining source of the deposit is the land, from 

 which clastic material might be brought by the wind or by the rivers. 

 If brought by the wind and deposited far enough from shore to be 

 free from coarse material, the deposit would not have a circumscribed 

 areal distribution. Such a restricted distribution is, however, possi- 

 ble if the material has been supplied by the rivers. If carried into 

 the sea, it may be deposited in quiet water, and this may produce 

 such a fine-grained rock as the waterlime, which is free from coarse 

 elastics. Such regions of deposition would be found either far out 

 at sea where all of the nearer-shore, coarser elastics were absent, or 

 else near the shore, but in sheltered bays. If these river-borne 

 muds were not carried into the sea, then they must have been de- 

 posited on land in the river flood-plains. 



We may consider for a moment the possibility of this formation 

 having been deposited at a sufficient distance from land to allow of 

 the quiet accumulation of fine sediments, or else in sheltered areas 

 along shore. Such deposits at the present time are represented by 

 the blue or slate-colored muds, and these are the ones which are 

 spread over the floors of shallow seas and out to the edge of the 

 continental shelf. Murray and Renard (194) have estimated that 

 these muds cover 14,500,000 square miles of the ocean floor. An 

 average analysis shows the following composition: 



Ignition 5 . 60 CaC0 3 2.94 



SiC>2 64 . 20 Ca 3 P 2 08 i-39 



AI2O3 13-55 CaSC>4 0.42 



Fe 2 3 8.38 MgC0 3 0.76 



CaO 2.51 



MgO ' 0.25 100.00 



