364 Story of the Mississippi- Missouri. — Claypole. 
ing its continuance. Our first view of the palaeozoic ocean of 
North America is at the beginning of the following, the Ordo- 
vician, or as it is still called by many geologists, the Lower 
Silurian. The northern shore of this body of water was then 
the Laurentide mountains of Canada, ranging from the Atlan- 
tic coast in Labrador along the northern side of the great lakes 
to the southwest and there turning sharply northwestward to 
the shore of the Arctic ocean near the mouth of the Mackenzie 
river. Its eastern limit lay, so far as can at present be deter- 
mined, along the line of the Blue Ridge in Pennsylvania and 
Virginia ; thence northeastward through New England and 
Lower Canada to Newfoundland ; and southwestward through 
Tennessee to Alabama. Between the northern part of this 
coast-line and the Laurentides along the present St. Lawrence 
valley there lay a long depression occupied during a part at 
least of the era by the sea. Southward from Alabama all 
traces are lost, the old coast-line being deeply covered by 
later deposits. 
Regarding the southern shore of this palaeozoic ocean noth- 
ing is yet known. Its waters may have extended over the 
present gulf of Mexico into Central and South America. But 
in the total absence of evidence all attempt at delineating it 
would be mere guessing. Nor are we in much better position 
in attempting to define its western shore. Where now stand 
the various ranges of the western states it is certain that large 
areas were covered with water. But a few ranges of highlands 
raised their heads above the ocean, forming an imperfect bar- 
rier on that side. 
These are all the relics that we have yet succeeded in dis- 
covering of the boundaries of this great palaeozoic ocean. But 
that others existed and that these were then more extensive 
may be readily inferred by the student of geology. The great 
masses of strata that lie in the west must have come from the 
destruction of pre-existing land and they indicate the past exist- 
ence of much more than the scanty fragments outlined above. 
Probably it was not continuous but consisted of separate and 
perhaps scattered reefs and islands.^ 
In the ocean basin thus formed the post-Cambrian deposits of 
1 Regarding the probable past existence of greater land surfaces as 
quarries from which the massive palaeozoic sediments of the east were 
derived see a paper by the writer in the American Naturalist for De- 
cember, 1887, entitled " Materials of the Appalachians." 
