THE ADIRONDACK MOUNTAINS 49 



trilobites. All the organisms mentioned lived in the sea water, and 

 if land forms existed we know nothing of them. It must be borne 

 in mind that not a single species of that time lives today, so complete 

 has been the evolutionary change since Ordovician time. Certain 

 remarkable classes of animals, like the graptoHtes and trilobites, 

 which often fairly swarmed in the Ordovician sea, have been wholly 

 extinct for millions of years. 



At or toward the close of the Ordovician period, a great com- 

 pressive force was brought to bear upon a thick mass of sediments 

 which reached from north of New England to Virginia, the strata 

 having been highly folded, tilted and elevated far above sea level 

 into a magnificent mountain range known as the Taconic moun- 

 tains. Accompanying this notable physical revolution, the Adiron- 

 dack region appears also to have been raised well above the sea 

 though without any folding of the strata. The facts that no post- 

 Ordovician strata occur in or close to the Adirondacks and that 

 there is a distinct break in the sedimentary record between the Ordo- 

 vician and succeeding Silurian in central New York, strongly sup- 

 port the idea of Adirondack uplift at that time. At about the close 

 of the Ordovician or the opening of the Silurian period, therefore, 

 all northern New York was dry land with the great Taconic range 

 standing out prominently just to the east. 



Later Paleozoic history. Early in the Silurian period, sea water 

 encroached upon central New York and kept that area submerged 

 during most of the period. How much, if any. of the Adirondack 

 region was covered by the Silurian sea ? The total absence of any 

 rocks later than the Ordovician shales around the Adirondacks and 

 just across the St Lawrence valley strongly suggests that those 

 areas continued as dry land not only during the Silurian but ever 

 since, except for a very brief local submergence after the Ice Age. 

 For the southern Adirondacks the case is somewhat different. Ex- 

 tensive outcrops of Silurian strata just south of the Mohawk river 

 make it certain that these rocks formerly reached farther north- 

 ward, having since been removed by erosion. But how far north- 

 ward they extended is a very difficult matter to decide since not a 

 scrap of Silurian strata now occurs north of the Mohawk river. 

 All we can say is that the Silurian sea probably overspread the 

 southern border of the Adirondacks, the sediments there deposited 

 having since been removed by erosion. Abruptly truncated (by 

 erosion) and only slightly tilted Silurian strata on the south side of 

 the Mohawk valley render this view practically certain. In a simi- 

 lar manner the succeeding Devonian sea may have spread over a 



