i882. S3 Trans. N. V. Ac. Set. 



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 granies 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 

 Jieight. 



The Huronian series, which follows the Laurentian, consists mainly 

 of slaes, 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 

 h'ghlands, 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 m 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 m 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 entirely of the hard parts of animals which inhabited 

 ihe sea. These accumulated slowly age afier age, in water so quiet 



