Story of the Mississippi-Missouri .— Claypole. 365 
the palaeozoic era were laid down in succession. Avoiding details 
these were the Ordovician, the Silurian, the Devonian and the 
Carboniferous. Some of these, especially the earlier ones, ex¬ 
tended over the whole region, while owing to local conditions 
others were confined to much smaller areas. The vast masses 
of sediment stretching from the Appalachians to the Rocky 
mountains and from the Laurentides to the gulf of Mexico are 
composed of the wash and waste of their contemporary lands. 
Rivers that have long ago disappeared bore down their tribute 
of sand and mud and strewed it over the floor of the ocean, to 
to be further distributed by its waves and tides. The greater 
part by far of this sediment was scattered along the Atlantic 
border where a continuous depression of the ocean bed was in 
progress. This is amply proved by the fact that in some 
places forty thousand feet of strata have been accumulated 
bearing through their whole mass the marks of shallow water. 
Whether this depression was the cause or the consequence of 
the deposit is still, after years of investigation, a moot point 
in geology. But certain it is that the two proceeded contem¬ 
poraneously and that the ultimate extent of the depression 
was not less than seven or eight miles, over an area at least 
three hundred miles in width and running along the whole 
length of the coast line traced above. 
This trough is filled with the shales, sandstones and con¬ 
glomerates that resulted from the weathering and wasting of 
a passed-away continent. It contains but few limestones. 
Limestones are the result for the most part of deep sea condi¬ 
tions and accordingly as soon as we recede from the coast we 
observe a change in the nature of the strata. The same bed 
which in Pennsylvania is a shale or even a sandstone becomes 
a limestone when it is traced into Indiana and Illinois. Thus 
the Utica and Hudson-River shales of the East become lime¬ 
stones in the interior basin. Even the sandstones of the Coal- 
Measures are in like manner represented by limestones in the 
midland states. Their thickness also diminished at the same 
time. Instead of the six, seven or eight miles that measure 
them in New York, Pennsylvania, Virginia, Tennessee and 
Alabama we find the whole series so reduced as not to exceed 
five thousand feet in depth. So great is the difference both in 
mass and in material that in many cases it has proved impos¬ 
sible to identify the strata in the west with their counterparts 
