392 



THE CAMBRIAN. 



lBULL.81. 



the nature and relative situation of the minerals in the United States, while they are 

 certainly the most extensive of any field yet examined, may perhaps be found to be 

 the most correct elucidation of the general exactitude of that theory as respects the 

 relative position of the different series of rocks. 



On the map four great classes of rocks are recognized: Class I, Primi- 

 tive rocks; II, Transition rocks; III, Secondary rocks; IV, Alluvial 

 rocks. The presence of rock salt in the southern Appalachian is rec- 

 ognized by a separate color. 



In the text 1 the Primitive rocks are divided into fourteen varieties, 

 the Transition into five varieties, the Secondary into twelve, and the 

 Alluvial into seven. In traversing the country Mr.Maclure noticed the 

 presence of any one of these rocks at their various outcrops, and 

 thus correlated the geological formations that presented similar 

 characters. In the second edition of his work 2 two methods of ex- 

 amining and correlating the rocks are given : 



The first, the accurate investigation of a small portion of the sur- 

 face, describing exactly the different rocks with their immense variety 

 of arrangement and position of their component parts, detailing the 

 changes, accidental or natural, constantly occurring in their relative 

 situation, and endeavoring to reduce the whole to some regular series 

 of arrangement. This method necessitates the reunion of a great num- 

 ber of those portions before any correct general idea can be formed, 



The second, beginning with the great outline, traces the limits which 

 divide the principal classes of rocks and their relative situation and 

 extent. Mr. Maclure favors the second, stating: 



In tracing the outlines of the different formations in most countries there is less 

 confusion and embarrassing description necessary ; the limits once ascertained, a few 

 pages define the boundaries and explain the relative situations to the comprehension 

 of every reader. 3 



Eaton. — After Mr. Maclure, Prof. Amos Eaton was the most impor- 

 tant factor in the shaping of the methods of geologic work in America. 

 By his various publications and his influence as a teacher he outlined 

 the methods and formulated the general principles upon which geologic 

 investigation was carried forward for nearly two decades. The correla- 

 tions made in his earlier works were based entirely upon the lithologic 

 characters of the rocks. Believing in the Mosaic cosmogony, he ex- 

 plained the phenomena he observed in the structural relations of the 

 rocks by the giving way of the foundation of the Old World prior to 

 the flood. He says : 4 



While this tremendous crash of nature was going on, scales of various thicknesses 

 from the various strata were shot up, detached, and broken, which gave formation to 

 our surrounding hills, the ragged cliffs of the Catskill, and the bleak brow of the 



l Op cit.,p.4l2. 



Observations on the Geology of the United States ; with remarks on the probable effects that may 

 be produced by the decomposition of the different classes of rocks on the nature and fertility of soils. 

 Am. T'hil. Soc. Trans., vol. 1, new ser., 1817, pp. 1-91. 



3 0|». cit.,p.9. 



4 An index to the Geology of the Northern States, with a transverse section from Catskill Mountain 

 to the Atlantic. Leicester, 1818, pp. 48, 49. 



