CARTOGRAPHIC METHODS. 583 



generally late Mesozoic or Tertiary. A wholesome result of this would be 

 the increased attention that the eastern student would give to these later 

 divisions of time on the Atlantic slope. At present he is disposed to neglect 

 them, unless for special reasons he is particularly engaged in their study on 

 the coastal plain, and even then their moderate thickness fails to give a 

 properly impressive measure of the time that they represent. This would be 

 better emphasized in the study of the denudation that these periods witnessed. 



But the real value of a colored map showing the dates of the topographic 

 forms would be found in the attention that it would call to the full meaning 

 of the forms themselves. The current understanding of our Atlantic slope 

 would certainly be greatly modified. Particularly in the case of our moun- 

 tains, and the valleys among them, are current ideas vague and inaccurate. 

 In the first place, it is too often tacitly assumed that the mountain ridges of 

 the Appalachians are simply the unconsumed residuals of the original post- 

 Carboniferous folding and upheaval. It may be asserted with much confi- 

 dence that if there had been no uplift since that time there would be no 

 mountains now. The Appalachians in Pennsylvania at least have been 

 rubbed out once certainly, and perhaps twice ; and what we now see may be 

 truly likened to a cameo ; a plateau and a series of ridges wrought out of a 

 low, even surface by the wasting of its weaker parts after it had been moder- 

 ately uplifted. Instead, then, of looking at our mountains as remnants of an 

 unknown mass worked on by erosive forces during unknown times, they may 

 be much more precisely understood. They are essentially the products of 

 Tertiary erosion on an uplifted Cretaceous peneplain of moderate relief. 

 The pre-Cretaceous forms are in nearly all parts lost ; the post-Tertiary work 

 is in nearly all places insignificant. Our topography is, for the most part, 

 a Tertiary product. 



A map of Pennsylvania colored to show the dates of origin of its topo- 

 graphic forms would possess a considerable area of Cretaceous greens, in- 

 cluding here all the uplands. If any hills are found on the uplands they 

 might be dotted green, to indicate that they were not completely worn down 

 to baselevel in Cretaceous time. Another color of broad application would 

 be the general Tertiary yellow. It would spread evenly over the lowlands, 

 but would fall in dots or lines on the slopes descending from the Cretaceous 

 uplands. Only the narrow trenches would be marked by strips of post- 

 Tertiary color. With increase of scale, the more minute features might find 

 recognition ; and, in the end, the topographic map might be nearly as much 

 of a patch-work of colors as the geological map is now. Nothing will con- 

 tribute more to the realization of this end than the preparation of contoured 

 topographic maps, such as those of the United States Geological Survey, so 

 often referred to in this essay. Whether of final accuracy in all regions or 



