230 
TRANSITION ROCKS. 
CHAPTER XXL 
GEOLOGY OF THE UNITED STATES. 
TRANSITION ROCKS. 
Definition.— Clay Slate.— Its Distribution.— Transition Lime* 
stone.— Mr. M'Clure's Description of this Formation. — Mr. 
Featherstonhaugh's Old Red Sandstone.— Its Geological Dis- 
tribution. 
We have already remarked that the transition 
rocks are those that rest on the primary, and, as 
they seem to pass into them, they are called transi- 
tion^ from the Latin words trans and eo, to go or pass 
over. They are sometimes called intermediate ; 
but as it is difficult to tell where they begin or where 
they end, some geologists, as Professor Sedgwick, 
Mr. Brown, &c., merge them either wholly in the 
primary, or partly in that and partlj'' in the second- 
ary classes. As the earliest of the transition class, 
argillaceous slate, does contain fossil* organic re- 
mains, we find it necessary, from our definition of 
primary rocks, and far more suited to the existing 
order of nature, to place this rock, which has gener- 
ally been arranged among the latter, among the 
transition series. But, without entering on the un- 
profitable dispute whether clay slate should belong 
to the transition, or whether, indeed, there should 
be any transition class or not, our object will be at- 
* " This is the oldest system of strata in which organic re- 
mains are certainly known to occur ; and it may surprise the 
speculators in cosmogony to hear that these, the most ancient 
forms of life known to us, should be not plants, but animals," 
&c.— Philips's Geology, vol, i., p. 128. 
