been 40, 
464 J. LeConte—Formation of the 
trations taken from the history of mountain chains in North 
erica. 
Appalachians.—The area now occupied by the Appalachian 
chain was, during the Silurian and Devonian ages, the eastern 
margin of the bed of the great interior Paleozoic sea. During all this 
time the whole Paleozoic sea, but especially this eastern margin, 
received sediments from a continental mass to the northward 
(the Laurentian area), and also especially from a continental mass 
to the eastward. Besides the marks of shore deposit found 
abundantly in the Appalachian strata, other evidences are daily 
accumulating that the area to the east of the Appalachian 
chain, left blank in the geological map of the United States im 
Dana's text book—the so-called primary or gneissic region 0 
the Atlantic slope—is Laurentian, and therefore was probably 
land during the Paleozoic times. The size of this eastern contl- 
nental mass it is impossible for us now to know, as it has been 
partly covered by later deposits, and perhaps even partly cov- 
the sea; but, judging from the quantity of sediments 
carried into the Paleozoic sea. and especially from the thick- 
ness of the sediments (30,000 feet) along its eastern margin, 
derived probably wholly from this source, it must have been 
bea large. Z 
t the end of the Devonian age, much of the middle portion 
of the interior Paleozoic sea was upheaved and became land 
(see Dana’s map, Manual, p. 183); and the Appalachian area 
now became alternately a coal marsh and an estuary emptying 
into the sea southward. Into this estuary or marsh, during the 
whole Coal period, sediments were brought from land north, 
east, and west, until 10,000 feet more had been deposi 
The subsidence of the Appalachian area, therefore, must have 
een 40 t i 
_ During the Coal period, therefore, the Appalachian region 
was still nearly on a fore with the sea. So far from being 4 
