1855.] 



GEOLOGY IN AMERICA. 



359 



forms the conditions of the time allowed, we may reasonably infer that 

 there m.ay have been in Azoic times marine plants and plant infusoria, 

 forms adapted to aid in the earth's physical history ; and this vegeta- 

 tion may have long preceded animal life on the globe. 



After these general remarks on the divisions of geological time, I now 

 propose to take up the characteristic features and succession of events 

 in Amei'ican Geology. 



In the outset we are sti'uck with the comparative simplicity of the 

 North American continent, both in form and structure. In outline it is 

 a triangle, the simplest of mathematical ingures ; in surface it is only 

 a vast plain, lying between two mountain ranges, one on either border ; 

 tlie Appalachian, from Labrador to Alabama on the east, the Rocky 

 Mountains on the west; and on its contour it has water, east, Treat, 

 north, south. 



Observe, too, that its border heights arc proportioned to the size of 

 the oceans. A lofty chain borders the Pacific, a low one the naiTow 

 Atlantic ; while the small Ai'ctic is faced by no proper mountain range. 



This principle, that the highest mountains of the continents face the 

 largest oceans, is of wide application, and unlocks many mysteries in 

 physical geogi'aphy. South America lies between the same oceans as 

 North America ; it has its eastern low range, its western Andes ; and 

 as the oceans widen southward, the continent is there pinched up to 

 almost a narrow mountain ridge ; it differs from North America in hav- 

 ing a large expanse of ocean, the Atlantic, on the north, and correspon- 

 dingly it has its northern mountain ridge. The world is full of such 

 illustrations, but I pass them by. 



Tliis simplicity of ocean boundary, of surface features and of out- 

 line, accounts for the simplicity of geological structure in North Ame- 

 rica ; or we may make the wider statement, that all these qualities are 

 some way connected with the position and extent of the ocean, they 

 seeming to point to the principle that the subsidence of the oceanic 

 basins has determined the continental features. Aiuerica has thus the 

 simplicity of a single evolved result. Europe, on the contrary, is a 

 world cf complexities. It is but one coi'ner of the oriental continent, 

 which includes Europe, Asia, and Africa, and while the ocean bounds 

 it on the north and west, continental lands enclose it on the south and 

 east. It has ever been full of cross purposes. American strata often 

 stretch from tlie Atlantic west beyond the Mississippi ; and east of the 

 Rocky Mountains, it has but one proper mountain range of later date 

 than the Silurian. Europe is much broken up into basins, and has 

 mountains of all ages ; even the Alps and Pyrenees are as recent as the 

 tertiary. This wide contrast accounts for the greater completeness or 

 generality of American revolutions, and the more abrupt limits of 

 periods and clearer exhibition of many geological principles. 



The geological structure of this country has been made known through 

 the combined researches of a large number of investigators. The names 

 of Maclure, SiUiman, Eaton, lead off the roll. Hitchcock, the Profes- 

 sors Rogers, the well-known Geologists of the New York Survey, Owen, 

 Percival, Morton, Conrad, Tuomey, and many others, have contributed 

 to the collected results. Yet the system may be said to have been 

 mainly laid open by three sets of observers — Morton and Conrad for the 

 Cretaceous and Tertiary, the New York Geologists for the Paleozoic 

 strata ; and the Professors Rogers for the Carboniferous beds and the 

 Appalachians. 



The succession of Silurian aad Devonian rocks in the State of New 

 York is the most complete in the country, and it was well for the science 

 that its rocks were so early studied, and with such exactness of detail. 

 The final display of the PaloDontology by Mr. Hall has given great pre- 

 cision to the facts, and the system has thereby become a standard of 

 comparison for the wliole country, and even for the world. This accom- 

 plislied, the carboniferous rocks were still to be registered, and the 

 grand problem of New England Geology solved. Tlie Professors Rog- 

 ers, in the survey of Pennsylvania and Virginia, followed out the suc- 

 cession of strata from the Devonian througli the Coal period, and thus 

 in a general way completed the series. And more than this, they un- 

 ravelled with consummate sldll the contortions among the Appalacliians, 

 bringing order out of confusion, and elucidating a principle of moun- 

 tain-making which is almost universal in its application. They showed 

 that the Silurian, Devonian, and Carboniferous strata, which were ori- 

 ginally laid out in horizontal layers, were afterward pressed on to the 

 north-westward, and folded up, till the foMs were of mountain height, 

 and thus the Appalachians liad tlieir origin ; and also, that by the es- 

 caping heat of those times of revolution extensive strata were altered 

 or even crystallized. 



This key soon opened to us a knowledge of New England Geology, 

 mainly through tlie labours of Mr. Hall, ami also Professor H. D. 

 Rogers, following up llie survey of Pros. Hitchcock : and now tliosc so- 



called primary rocks, granite, gneiss, mica-schist, and crystalline lime- 

 stones, once regarded as the oldest crystallizations of a cooling globe, 

 are confidently set down as for the most part no older than the Silu- 

 rian, Devonian, and Carboniferous beds of New York and Pennsylvania. 



Let us now briefly review the succession of epochs in American 

 Geological history. 



The Azoic age tended, as was observed, in a period of extensive 

 metamorphic action and disturbance ; in other words, in a great revo- 

 lution. At its close, some parts of the continent were left as dry land, 

 which appear to have remained so as a general thing in after times ; 

 for no subsequent strata cover them. Such are a region in Northern 

 New York, others about and beyond Lake Superior, and a large terri- 

 tory across the continent from Labrador westward, as recognized by 

 Messrs. Whitney and Foster, and the geologists of Canada. 



The Silurian or Molluscan Age next opens. The lowest rock is a 

 sandstone, ono of the most widely spread rocks of the continent, 

 stretching from New England and Canada south and west, and reach- 

 ing beyond the Mississippi — how far is not known. And this first leaf 

 in the record of life is like a title-page to the whole volume, long after- 

 ward completed : for the nature of the history is here declared in a 

 few comprehensive enunciations. 



1. The rock from its thin, even layers and very great extent shows 

 the wide action of the ocean in distributing and working over the sands 

 of which it was made ; and the ocean ever afterward was the most 

 active agency in rock-making. 



2. Moreover, ripple^marks such as are made on our present sea- 

 shores or in shallow waters, abound in the rock both through the east 

 and west, and there are other evidences also of moderate depth and of 

 emerged land. They all announce the wonderful fact that even then, 

 in that early day, when life first began to light up the globe, the 

 continent had its existence — not in embryo, but even of full-grown 

 extent, and the whole future record is but a working upon the same 

 basis and essentially within the same limits. It is true that but little 

 of it was above the sea, but equally true that little of it was at gi-eat 

 depths in the ocean. 



3. Again, in the remains of life which appear in the earliest layers 

 of this primal rock, three of the four great branches of the animal 

 kingdom are represented : Molluscs, Tribolites among .Vrticulates, Co- 

 rals and Crinoids among Radiates — a sufficient representation of life 

 for a title-page. The New York beds of this rock had afforded only a 

 few Molluscs, but the investigations of Owen in Wisconsin have added 

 ihe other tribes ; and this diversity of forms is confirmed by Barrande 

 in his Bohemian researches. Among the genera, while the most of 

 them were ancient forms that afterwards became extinct — and through 

 succeeding ages thousands of other genera appeared and disappeared — 

 the very earliest and most universal was one that now exists — the genus 

 Lingula — thus connecting the exti-eraes of time, and declai'ing most im- 

 pressively the unity of creation. Mr. Hunt, of the Canada Geological 

 Survey, recently discovered that the ancient shell had the anomalous 

 chemical constitution of bones, being mainly phosphate of lime, and 

 afterward he found in a modern Lingula the very same composition — 

 a further announcement of the harmony between the earliest and latest 

 events in geological history. 



The earliest sandstone, called in New York the Potsdam sandstone, 

 and the associate calciferous sand-rock, mark off the First Period of the 

 Molluscan Age, the Potsdam Period, as it maybe called. 



Next followed the Trenton Period — a period of limestones (the Tren- 

 ton limestone among them) equal to the earlier beds in geographical 

 limits, and far more abundant in life, for some of the beds are literally 

 shells and corals packed up in bulk ; yet the species were new to the 

 period, the former life having passed away ; and even before tho 

 Trenton period closes, there were one or two epochs of destruction of 

 life, followed by new creations. Tlio formation of these limestone 

 beds indicated an increase in the depth of the continental seas — an in- 

 stance of the oscillation of level to which tho earth's ci-ust was almost 

 unceasingly subject through all geological ages until the present. 



After the Trenton period, another change came over the continent, 

 and clayey rocks or shales were formed in thick deposits in New York 

 and to the south — tho Utica Slate and Hudson River Slinles — while 

 limestones were continued in the west. This is the lliidton Period, 

 and with it the Loicer Sihiriiin closed. 



The seas were then swept of their life again, and an abrupt transi- 

 tion took place both in species and rocks. A congloMicrafc covered a 

 large part of New York and tho States south, its coarse material 

 evidence of an epoch of violence and catastrophe ; and with this depo- 

 sit the Upper Silurian begins. 



Tlio Upper Silui-ian has also its three great peril"! 



