VALLEY REGION ; CHARACTERS OF THE ROOKS. 147 
many places in Alabama, and particularly in the region 
covered by this map, where the rock beds have been com- 
pletely overturned, so that the older beds are on top of the 
younger. It would often be impossible to determine the 
relative ages of these rocks by their physical characters, and 
where they have been overturned their relative position 
would of course, be absolutely misleading if we judged by 
tne stratigraphical position alone; but as each of these great 
divisions has its characteristic fossils, these become in many 
cases our safest, and sometimes our only trustworthy guides 
in determining the age of the rocks in which they are im- 
bedded. 
Since all these rocks have been formed either out of the 
detritus or waste of previously existing land masses (con- 
glomerates, sandstones, grits, shales and slates), or through 
the agency of living organisms, (limestones, flinty or cherty 
matters, and coal and all forms of bituminous matters), one 
would naturally think that it would be impossible to dis- 
tinguish one sandstone or one limestone from another, or in 
other words to distinguish one of our geological formations 
from ‘another by its lithological or rock characters. As 
a matter of fact, however, the field geologist, after a very few 
weeks or months of practice, learns to distinguish the 
different formations by their rocks, and hence the lithologi- 
cal characters are of almost equal value with the fossils in 
- classifying our rock formations, and inasmuch as the fossils 
are nowhere very abundant, in the great majority of cases 
we make use of the lithological characters alone in studying 
and identifying the different geological formations. 
It is easy to see that it is nearly impossible to describe 
the rocks of these older formations in terms which will en- 
able the inexperienced observer to identify them, yet a short 
account of the prevailing characteristics of the rocks is nec- 
essary to the full understanding of the description of their 
distribution in the valleys. It must, however, be constantly 
borne in mind that the characters of the rocks of all these 
formations vary with the geographical locality, they being 
generally coarser in texture and more siliceous towards the 
east than further west. Thus in the Cambrian formation 
there are in the Coosa Valley beds of immense thickness of 
