16 OUTLINE OF THIS PAPER. [bull. 81. 



Lower and Middle Cambrian time and that the Cambrian Sea did not enter npon it 

 until the beginning of Upper Cambrian time. A map based (PI. in) upon this view 

 illustrates the supposed land area at the beginning of Cambrian time. 



The base of the group is stated to be where the oldest known fauna that is referred 

 to the Cambrian fauna occurs. The summit of the group is the transition from the 

 Cambrian to the Ordovician fauna, or in the New York section between the Pots- 

 dam sandstone and the superjacent Calciferous formation. The rocks or sediments 

 are described and the conclusion reached that there were geographic areas in which 

 different types of sedimentation prevailed, and that as a whole the sediments were 

 deposited in both shallow and deep waters on an ocean bottom that was being slowly 

 depressed in relation to sea level. 



The study of the fauna indicates that the group may be conveniently divided into 

 three divisions, lower, middle, and upper. The evidence of the sedimentation and 

 organic remains proves the presence of geographic provinces in Lower Cambrian time 

 that became more strongly differentiated in Middle Cambrian time and less strongly 

 marked in Upper Cambrian time. 



The chapter concludes by notes on the Cambrian rocks of Great Britain, Europe, 

 China, India, Australia, and South America. 



Chapter V deals with problems for investigation and settlement which are taken 

 up, for the local problems, in the same order of presentation as the: matter in chap- 

 ters ii and IV. The general problems respecting our knowledge of the Cambrian 

 group, as a whole, or in large parts, are discussed separately, as are the problems of 

 nomenclature and classification. The latter includes a statement of the problems 

 relating to the name and the limits of the group. 



Chapter VI is devoted to the study of criteria and principles used by authors in the 

 correlation of the various parts composing the Cambrian group, with observations 

 on some methods of correlation. The first portion includes the historical notes, and 

 is a review in chronologic order of the principles of correlation used by authors in the 

 United States and Canada, and observations by authors upon correlation of American 

 with European formations. This review shows that many authors have expressed 

 sound views upon methods of correlation and that, with the exception of the addi- 

 tions made since the theory of evolution began to influence paleontologists, the 

 addition to methods has been relatively small. Under the heading of methods of 

 correlation, correlation by superposition, organic remains, lithologic characters, 

 and unconformity are treated of at some length. Under organic remains are con- 

 sidered life zones, stage of evolution, life history, contemporaneity, homotaxis, and 

 percentage of species. Reference is also made to homogeny and topographic features 

 as elements entering into the problems of correlation. 



