152 IOWA ACADEMY OP SCIENCES. 



connected whole a number of views which have long been 

 known, somewhat vaguely as a rule, perhaps, yet which are in 

 fact, to a certain degree at least, the real foundation of 

 systematic geology. It is the christening of the scheme with a 

 title in which the governing causes of sedimentation are recog- 

 nized, in which the elements rendering possible any systematic 

 arrangement are brought into due jn'ominence, and in which an 

 old principle is greatly extended in its application, and is 

 relieved of much of that which has so long overshadowed it. 



In general geological classification, about the only attempt in 

 which the orotaxial principle has shown itself in the past, 

 is in demarkation of the grand divisions or systems, and the 

 events are commonly referred to as geological revolutions. 

 The nearest approach to the practical application of the idea, in 

 some of its phases, has been by Irving,* in his work on 

 the pre-Cambrian crystallines of the Northwest, in which 

 unconformities are given great prominence; by McGee,t in 

 his investigations of the coastal plain deposits of the middle 

 Atlantic slope, in which similarity of origin, or homogeny, is 

 the governing factor; and by DavisJ and others in their physi- 

 ographic work, in which periods of base-leveling are made the 

 all-important features in the cycles of land degradation and the 

 consequent sedimentation in adjoining seas. 



CONCLUSIONS. 



Proceeding upon the suggestions that have just been made, 

 the principles of general correlation may be more clearly shown 

 by the construction of a chart (plate vi) representing a section 

 across the North American continent, in an east and west 

 direction, as for instance from Richmond to San Francisco. In 

 a diagrammatic representation of this kind, the geographic 

 provinces are cut off by vertical lines, and the geological 

 systems by horizontal ones, the latter being separated by dis- 

 tances approximately proportional to the estimated time inter- 

 val. The skeletal chart stands for continuous and uninter- 

 rupted geological history of the continent and the stratigraphi- 

 cal succession from the earliest to the latest formations. In 

 the proper places are indicated some of the principal physical 



*IJ. S. Geol. Sur., 7th Ann. Rept., p. 378, 1888. 

 +Am. Joup. Sci., (3), Vol. XL, pp. 36-41, 1890. 

 *Nat. Geog. Mag., Vol. I, pp. 183-253, 1889. 



