8 THE DEVONIAN AND CARBONIFEROUS. [bull. 80. 



rately the evidence from vertebrate fossils and the evidence from fossil plants as to 

 all the systems in which they are found ; and there will he prepared in connection 

 with the work a thesaurus of North American stratigraphic terminology. 



The work has been placed under the general charge of Mr. G. K. Gilbert, and a 

 number of specialists to assist him have already been selected from the various divi- 

 sions of the Survey. 



Each of the systems indicated above was assigned to a paleontologist 

 or a geologist for treatment, and several conferences were held for the 

 purpose of developing a definite plan of work. Eventually the plan was 

 formulated as follows, being incorparated in a circular letter addressed 

 the Director to the several specialists chosen for the work in February, 

 1888: 



PLAN FOR THE DISCUSSION OF AMERICAN GEOLOGIC SYSTEMS. 



(1) It is proposed to prepare an essay on each of the following American geologic 

 systems, namely: (1) Quaternary, (2) Plio-Miocene, (3) Oligo-Eocene, (4) Cretaceous, 

 (5) Jura-Trias, (6) Permo-Carboniferous, (7) Devonian, (8) Silurian, (9) Cambrian, 

 (10) x y z, (11) Archean. 



The " Congress" committee of the American Association for the Advancement of 

 Science at a recent meeting resolved (in effect) that ' 4 systems are determined primarily 

 by fossils, secondarily by structure." This series of essays is planned on the assump- 

 tion that for purposes of correlation the most important fossils are marine inverte- 

 brates. The evidence from vertebrates and that from plants will be discussed each 

 by an appropriate specialist, but this arrangement does not preclude their considera- 

 tion in the essays on individual systems. 



(2) Each essay should show how the system of which it treats has been paleonto- 

 logically and stratigraphically delimited in North America, and should recite and 

 discuss the facts and principles on which such delimitation is based. 



(3) Each essay should show into what series (major subdivisions) the system has 

 been divided in various parts of North America, and on what facts and principles 

 the division has been based. If these subdivisions are not uniform in all parts of 

 the continent the various areas of exposure should be classified in provinces, and 

 the essays should show whether and to what extent the series of the several provinces 

 can be correlated with one another. 



(4) Each essay should show whether and to what extent the subdivisions of the 

 system in any or all of its American provinces can be correlated with the subdivis- 

 ions of the system in Europe. 



(5) Each essay should be prepared with the aid of a comprehensive review of the 

 pertinent literature, so as to constitute a summary of the material at present avail- 

 able for the major taxonomy of the system. 



(6) The names of systems in (1) are provisional. Each essay should consider the 

 question of names for system and series. 



The number of systems is likewise provisional, and it may eventually appear that 

 those enumerated in (1) are not coordinate. It was necessary to prepare a scheme 

 in order to apportion the work of assembling the facts, but after these have been 

 assembled, their discussion may lead to an improved scheme. Provision will be 

 made for such discussion after the series of essays has been prepared. 



(7) The general purpose of the preparation of the series of essays is threefold : 

 first, to exhibit in a summary way the present state of knowledge of North American 

 geologic systems ; second, to formulate the principles of geologic correlation and 

 taxonomy; third, to set forth from the American standpoint the possibility, or the 



