W. A. Richardson—The Relative Age of Concretions. 119 
fossiliferous chert. On his hypothesis of chert formation, nodules 
originally existed as clobular segregations of silica jelly on the sea- 
floor. The fossiliferous character of the cherts is explained as due 
to the infalling of animal remains, which accumulated in these 
silica balls in the same way as in the surrounding rock. 
This hypothesis presents mechanical difficulties. A few 
experiments with any gel, even though it may contain a large amount 
of water, will suffice to show that, as soon as a gel has sufficiently 
coagulated to stand under its own weight, it offers considerable 
resistance to surface penetration. Heavy objects may be left 
indefinitely on its surface, and penetrate no further than a small 
initial indentation. 
Large heavy shells possess considerable surface, and this would 
prevent their penetration into the gel. With regard to minute forms, 
such as the tests of foraminifera, it is impossible to conceive how 
they could have penetrated the gel. They must also arrange them- 
selves in the same way as they are to be found in the surrounding 
rock. Yet the viscosity of the gel, even though one granted the 
possibility of penetration, would exercise a considerable effect on 
the arrangement of the contents. 
From such considerations it would seem'that our faith in the old 
argument of replacement is not considerably weakened by this 
ingenious suggestion. 
IV. Moprern OCCURRENCE. 
The occurrence, or non-occurrence, of a particular type of 
segregation in modern seas has often been employed as a subordinate 
criterion of relative age. It is, of course, an argument based on 
comparison. Certain types of concretion, e.g. manganese nodules 
and phosphatic segregations, are found. It, therefore, seems 
reasonable to expect careful search to reveal something of other 
concretions that are as abundant or more abundant in rocks of 
various geological ages. 
The argument has been frequently opposed. Sometimes on the 
ground that the specific eravity of the segregation would carry it 
deep into the deposits of the ocean-floor beyond reach. However, 
the list of the materials dredged in the numerous expeditions disposes 
of specific gravity as the cause of non-discovery. The list includes 
manganese and phosphate nodules; barytes, terrestrial rocks, 
including detrital flints, heavy animal remains, debris dropped by 
steamers, and meteoric granules." 
‘A more serious and less answerable objection lies in the possibility 
that physical conditions necessary for formation of the concretion 
in question do not pertain in the present ocean. The argument is, 
therefore, quite secondary, and must be used with due caution. It 
obtains weight only when a considerable body of other evidence 
points the same way. 
1 Murray & Hjort, The Depths of the Ocean, London, 1912, chap. iv. 
