552 NEW YORK STATE MUSEUM 



It has also been observed at other graptolite localities, for 

 example, the Normans kill shale on the north side of Mt Moreno 

 near Hudson, that the graptolite-bearing shale was enveloped 

 in greenish, hard silicioiis beds. The close stratigraphic con- 

 nection existing between these greenish and the graptolite 

 shales, and the absence of graptolites from the lithologically 

 similar black shale partings of the calcareous layers indicate 

 that both the former originated under closely similar condi- 

 tions. It suggests itself that the only change in the physical 

 conditions was in the swiftness of the current, the silicious 

 beds being deposited in a current which carried more material^ 

 while a slackening of the current allowed the slow deposition 

 of the fine argillaceous and carbonaceous mud which entombed 

 the graptolitic detritus. That the latter, in most of the Deep 

 kill beds, can not have been exposed to any turbulent wave or 

 current action, is clearly evinced by the retention of the most 

 delicate parts like the hydrocaulus. It is further clear that 

 the graptolites did not live continuously on the bottom where 

 they are now found, for they appear only intermittently and 

 then in vast multitudes and always in different associations. 

 The aspect of the paper-thin seams changes kaleidoscopically 

 from seam to seam; and often a surface will present nothing 

 but the spawn or only a certain growth stage of a single species. 

 These conditions of deposition, and similar ones in the TrentoD 

 and Utica zones argue that the fauna was, from time to time, 

 carried into this coastal region of the sea from an outside and 

 presumably pelagic region. 



The limestones which form considerable banks between the 

 graptolite beds are evidently not direct organic deposits or 

 shell heaps, but were either derived from the abrasion of a cal- 

 careous coast which furnished the fine calcareous mud, or were 

 direct chemical deposits such as are formed wherever decaying 

 organic matter furnishes the necessary carbonate of ammonia 

 to form calcium carbonate out of the gypsum contained in the 

 sea water. As the carbonaceous mud partings between the 

 calcareous layers indicate an oft-repeated interruption of the 



