METHODS OF PAL^OGEOGRAPHY 1147 



to be brought into the seas in which they were deposited. Thus, 

 as will be seen on the maps for the Lower Cambric (Fig. 264a), the 

 New England land barrier between the Atlantic and the Pacific ex- 

 tension in the Appalachian or Cumberland trough is much too nar- 

 row, while the width of that trough is also too small. The same 

 is true for the land-barrier in North Britain, between the Atlantic 

 and Arctic oceans. Since, however, the rocks carrying the faunas 

 of these two seas are found so much nearer together to-day than 

 was the case at the time of the deposition, such faulty construction 

 seems to be unavoidable. 



The overlap relations of marine strata are especially significant 

 as aids in determining old shore-lines, for such overlap of a later 

 over an earlier formation indicates, as a rule, that the point of 

 overlap is also a point on the shore-line of the earlier formation. 

 Due attention must here be given to the type of overlap (see Chap- 

 ter XVIII), and the fact must be borne in mind that minor over- 

 laps may also be developed upon an irregular sea-bottom, where 

 wave activity or current scour is active. 



Important factors that must not be overlooked in the construc- 

 tion of palnsogeographic maps are the nature of the sediment and its 

 source. Where coarse clastic sediments abound in the formation, 

 a land of sufficient size must have existed to furnish this sediment. 

 This is especially the case when the sediment consists of well-as- 

 sorted material, such as quartz-sand or pebbles, when it must be 

 remembered that such assorted material may represent only a part, 

 perhaps less than one-fourth, of the original rock which was its 

 source. In general, it may be said that much closer discrimination 

 between marine and non-marine sediments than has generally been 

 the case is necessary ; and the conditions of deposition must be 

 borne in mind, as well as the factors of marine and non-marine bion- 

 omy, and the effects of topography on currents and of both on 

 deposition, so that we may not again fall into the error of recon- 

 structing the area of former coral-rock formation as an arm of the 

 sea, one-half or one-quarter of a mile in width, and less than ten 

 fathoms in depth. 



When the science of Stratigraphy has developed so that its basis 

 is no longer purely or chiefly pakeontological, and when the sciences 

 of Lithogenesis, of Orogenesis and of Glyptogenesis, as well as of 

 Biogenesis, are given their due share in the comprehensive investi- 

 gation of the history of our earth, then we may hope that Palaeo- 

 geography, the youthful daughter science of Stratigraphy, will have 

 attained unto that stature which will make it the crowning attract- 

 ion to the student of earth history. 



