1882. ; 83 Trans. N. Y. Ac. Sct. 
clays of ancient times, the deposits of quiet waters off-shore ; while the 
marbles, which in some places form a considerable portion of the Lau- 
rentian series, are undoubtedly organic sediments that accumulated in 
relatively deep and quiet water by the slow process of growth and de- 
cay of animal structures. Thus the slates and the limestones are records 
of long continuance of quiet times and the absence of great high tides. 
Even the gneisses and grani’es are strata which must have been very 
different from such as would be formed by the impetuous rush to and 
fro, over the ocean’s shores, of a semi-diurnal wave hundreds of feet in 
height. 
The Huronian series, which follows the Laurentian, consists mainly 
of sla‘es, sometimes beautifully ripple-marked, and of beds of iron ore 
—all shore and shallow-water deposits, but speaking of quiet times 
and no high tides. 
The Cambrian rocks are but imperfectly shown on this continent, but 
they are all fine mechanical sediments, or earthy limestones, often fos- 
s'‘liferous, deposited along the Laurentian shore, but with an absence of 
all coarse material and cross-bedding, such as would be produced by 
rapid currents or violent ebbs and flows. 
In the Silurian series, which is here remarkably complete, we have a 
record that tells with great clearness the phys’cal, as well as the vital, 
history of the continent at that age. The Potsdam sandstone is an old 
beach spread over large areas of pre-existent land by a slow and quiet 
subsidence and an invasion of the Lower Silurian sea. The Laurentian 
highlands, the Adirondacks, etc., formed the shores of that sea, and 
‘the Silurian rocks were deposited in it. We know with considerable 
accuracy the boundaries of this sea, and can trace its shore line for a 
thousand miles as easily as we can that of the present Atlantic coast, 
and can study the littoral phenomena as satisfactorily. On the old 
beach, as on the new, gentle zephyrs covered the shelving bottom with 
ripple-marks ; the stems and fronds of sea-weeds are in places thickly 
interlaced, the beach-loving brachiopods strewed the shore with 
their whole or broken shells, and the boring annelids pierced the 
sand with innumerable holes. This automatic and indisputable record 
is so clear and simple that a child may read it, and it tells in unmis- 
takable language that in the beginning of the Lower Silurian age the 
littoral conditions were essentially the same as now, and that no high 
tides such as we have been considering could possibly have swept these 
shores. 
Above the Potsdam sandstone is spread a sheet of organic sediments 
—the great Trenton limestone group—-in places a thousand feet thick, 
composed almost entireiy of the hard parts of animals which inhabited 
the sea. These accumulated slowly age afer age, in water so quiet 
