INTRODUCTION. 



So many years have elapsed since the publication of the second volume 

 of the Palaeontology of New- York*, that it becomes proper and necessary 

 to preface the present volume with a short review of the results of the 

 great progress since made in our knowledge of the successive rock forma- 

 tions, not only ia regard to the higher groups, but also to those treated of 

 in the previous volumes of this work. 



Geological surveys, both public and private, have extended over large 

 areas of the United States ; and the very elaborate Geological Survey of 

 Canada, under the direction of Sir WiLLiAii E. Logan, has, more than all 

 others during this period, contributed to our knowledge of the older for- 

 mations. 



Among the important and interesting vestiges of ancient life in the 

 Potsdam sandstone, the investigations in the Canadian Geological Survey 

 have brought to light the existence of footprints and trails, produced 

 probably by crustaceans of several species, and some of them of large size. 

 These trails, extending over wide areas of smooth surface, and associated 

 with ripplemarks, furnish conclusive proof, if it were still needed, of the 

 shallow condition of the ancient sea. It is even certain that these tracks 

 are in part or wholly subaerial, since the action of the wind is clearly 

 shown upon the ripplemarked surfaces. 



Soon after the publication of the first volume of the Palaeontology of 

 New- York, the surveys of Dr. D. D. Owen in the northwest had made 



* ThU Tolame bean the date of 1863; bnt so far as regards the completion of the work, except the 

 preface, It was finished in 1851. 



[ Paleontology III.] 1 



