2 On the Physical Geology of the United States, §°c. 
is composed of sedimentary rocks* which have been formed by 
the aid of water, and organic secretion. 
Attempts have been made to measure the thickness of these 
rocks in Pennsylvania, Virginia, New York, and Ohio. The 
measurements in Pennsylvania give a thickness that excites as- 
tonishment, (seven to nine miles.) Those of central New York 
and Ohio, where the rocks are nearly horizontal, and undisturb- 
ed since their deposition, indicate’ a Jess thickness; but eight 
thousand feet in New York, and four thousand to five thousand 
feet in Ohio, is about the average thickness of these sedimentary 
rocks, and they are spread over an area of at least one million of 
square miles in the United States, exclusive of their extent in 
Texas, Mexico, and the British possessions. 
ese rocks vary in texture from a coarse conglomerate to the 
finest clays and shales. Several rocks of the same kinds, as 
sandstones, limestones, slates, coal, &&c. are repeated many times, 
yet each differs from the others of the same kind by some slight 
peculiarities, by which they may be recognized by careful study. 
Whatever may have been the causes of the formation and de- 
position of these rocks, it is evident that they have been so mod- 
ified in their action as to produce limestones at one time over 
nearly the whole of the vast area under consideration,—slates at 
another period, succeeded by sandstones,—again by limestones, 
slates, and sandstones,—and still again by conglomerates, slates, 
sandstones, coal, and iron ore, some of them alternating and re- 
peated many times. Similar causes have acted repeatedly over 
the same areas. 
The sand of the sandstones could be spread over such vast 
areas only by means of some cause tending to produce a broad 
current of moderate velocity ;+ the conglomerate would imply a 
* It is necessary here to make exceptions of Jimited tracts in Georgia, Missouri 
and Arkansas, where primary and metamorphic rocks occur; also the chipaiain 
region of northeastern New York. These masses of primary and metamorphic 
strata stand as geological islands surrounded by sedimentary rocks. 
1 Table showing the Transporting Power of Currents. 
VELOCITY OF CURRENTS. 
POWER OF TRANSPORT. —— ~~ i at P 
Wears awa fine, com: t, tough cla > ‘ 5 47 
Removes esand, . tie ms ° : ; - a 
sand as coarse as flax- seed, i - 8 0-45 
“fine gravel, ; 12} 068 
“ pebbles an inch in diamete: 24 = «|. «136 
“angular reasaente 2or3 inches i in fiessstes, 36 2-14 
