48 J. D. Dana on the origin of some of the 



Tertiary period which opened the JMammalian age, no marine foima- 

 tions were here made ; and there is hence no proof that in the long 

 interval between the origin of tlie trap dikes and the Glacial epoch, the 

 land of the region, or of any part of central New England, was at any 

 time under the sea. Whatever the fact, there must have been, during 

 the time that elapsed, a large amount of denudation over the region ; 

 so that West Rock, Pine Rock, Mill Rock and East Rock finally be- 

 came j^rominent above the plain, altliough much less so than now. 



3. General cnARACTER and results op the Post-tertiary period. 



Next came the Post-tertiary period, the last in Geological history. 

 In order to understand the following remarks it is necessary to bear in 

 mind that the Post-tertiary in America, as the writer has elsewhere 

 shown,* included three eras, corresponding to three great changes 

 of level over the northern portion of the Continent. 



1. The Glacial epoch ; when the land stood at a higher level than 

 now, and a universal glacier and a frigid climate covered the continent 

 north of the parallel of 40°, (not a sea with icebergs, as facts about 

 New Haven demonstrate.) 2. The Champlain epoch, an era of subsi- 

 dence ; when there was a sinking of the land below its present level, 

 resulting in a mild climate and a melting of the great glacier ; sub- 

 merging beneath the sea the land along the coast, and giving great 

 extent to lakes and rivers. 3. An epoch of elevation; bringing the 

 land up to its present level, and raising the submerged sea-shore and 

 river flats to a habitable and cultivable height, thus making them 

 available for man. The movements were up — down — up; up for the 

 Glacial era, down for the era following, and up again for the third or 

 finishing era. The origin of the features of the New Haven region 

 cannot be understood without keeping constantly in view these three 

 great movements of the land. In the first of these eras this region 

 stood probably one or two hundred feet above the level of the sea; in 

 the second sixty-five feet or more, and afterward forty and less, below 

 the present level ; and in the third it passed gradually to its present 

 condition. 



With reference to the question whether icebergs may not have been 

 the agent in the glacial era instead of glaciers, a single argument only 

 need here be brouglit forAvard. Icebergs, as is well known, are frag- 

 ments of glaciers broken off in the sea into which they descend ; and 

 the freight of stones and gravel they bear was received mainly when 

 they were in the glacier condition. The boulders of the Connecticut 



* Am. Jour. Sci., II, xxii, 325, 346, 1856, and Manual of Geology. 



