92 Marl of New Jersey. 
whiteness. Those who have seen it in the quarries, or noticed its 
vast masses not-distinctly stratified, and separated by beds of black 
flint, suppose these features essential to its external characters.” 
s * % % * 
‘¢ This idea, although tolerably correct, is not complete: for a long 
time its external and mineralogical characters were alone considered ; 
while its geological features, or those which belong to the formations 
(terrains) it embraces have been almost entirely overlooked, as well 
by geologists as by naturalists in gener al. 
_* The difficulty does not consist in recognizing beds of sie chalk 
containing black flint, but in detecting those deposits which want these 
external characters, and yet belong to the same age and formation. 
Now we determine a formation (that is to say, any combination of 
mineral substances which have been deposited at the same time,) by 
the position it occupies, relatively to the series which forms the crust 
of the globe; or in other words, by the kind of rocks or deposits 
which are constantly found above and below it: this is the geological 
character, or that of superposition, the most important of all when 
clearly ascertained. 
*¢ But in order to assign its proper place to any Brith: it must 
be so well characterized as to be known even when entirely insulated. 
The nature of the rock and the accompanying minerals may avail 
us in this inquiry, whence they are termed mineralogical characters: 
they are the most obvious, but at the same time the least essential 
characters, and most likely to deceive; and they, in fact, prevented 
our recognizing the chalk, (or rather the eretaceous formation,) in 
those places so familiar to us long before the time of M. Dufresnoy. 
“‘ A third series of distinctive characters is derived from. organic 
remains; these are called the — or organic characters of a 
formation. 
“Tt will now be understood what we mean by a chalk formation, 
or rather by a cretaceous formation: we must not picture to ourselves 
deposits wholly composed of white chalk ; but include in the desig- 
nation all those beds which occupy, among earthy strata, the same 
position as the chalk, enclosing the same characteristic fossils, and 
sometimes also presenting the same assemblage of mineralogical char- 
acters. We shall find cretaceous formations both black and yellow, 
en masse or stratified, with or without silex : we shall even see that 
these deposits are sometimes composed entirely of sand, and of sand- 
stone, without a particle of chalk, mineralogically speaking, or even 
of carbonate of lime.” 
