230 WHITMAN CROSS 
that ‘lithologic individual”? If any object be defined as having 
certain essential characters and it develops from experience that 
the object cannot always be identified by those characters, or, 
conversely, if it be found necessary to use supposedly unessential 
factors in its identification, there is something logically wrong 
in the conception of the object. 
Viewing sedimentary formations as documents of geological 
history, the deciphering of their complete record of events 
requires consideration of many things. The most striking fact 
in many cases is the lithologic character of the formation. 
From this character the mass receives its petrographic designa- 
tion, and from it some of the conditions of sedimentation and of 
local physiography at the time of deposition may be inferred: 
Often these factors permit further inferences as to the events 
preceding or following the deposition of a given stratum. But 
all the strictly lithologic characters have no chronologic signifi- 
cance. A sandstone, shale, conglomerate, or limestone may 
have been deposited at any time from the Algonkian to the 
Tertiary. As Mr. Willis clearly expresses it, ‘‘we now know 
that the physical characteristics of rocks are repeated from time 
to time, and are diverse in different provinces at the same time, 
and that therefore they do not afford criteria of contemporane- 
ous deposition.* Much more of the record of earth development 
may be found in the sedimentary deposit, however, if other data 
are considered. Such deposits consist usually of chemical pre- 
cipitates, fragments of older rocks, or of organic remains. 
Lithologically the source of materials for a clastic rock may 
be of no consequence. A conglomerate is a conglomerate, 
whatever the age may have been of the rocks which were 
destroyed to furnish sand or pebbles. But viewing the con- 
glomerate as a geologic formation, it is of great importance to 
note what earlier rock formations are represented in any given 
case. Even a single pebble or bowlder may throw more light 
upon the chronologic position of the conglomerate, and the 
on teien (Ss SEY /c 
