Kepresentation of the Dates of Topographic Forms by Colored 



Maps. 



Having opened this account of the topography of the Atlantic slope with 

 a statement of my conclusions, and having already made general review of 

 the more important results attained, it remains now only to consider an ap- 

 plication of the results that has been suggested to me by President Chamber- 

 lin. This is to represent the dates of the origin of topographic forms on 

 maps by colors, just as the dates of the deposition of certain geological struct- 

 ures are indicated by colors. The suggestion seems to me particularly valu- 

 able at this time, when the meaning of topography and the natural sequence 

 of the development of topographic forms are gaining much attention. Indeed, 

 when once suggested, the plan appears to be so useful, so natural, and so 

 needful that I can but wonder that it had not been proposed before. 



We are well accustomed to representing the dates of formations by colors. 

 We might indicate the dates of disturbances by colors also, and thus prepare 

 maps of our orogenic history. We may certainly, at least for the better- 

 studied parts of the world, indicate the geological dates of origin of our ex- 

 istent topographic forms, and, if not with entire certainty and accuracy, at 

 all events with a useful approach to accuracy quite as correct as the color- 

 ing of the early geological maps. Take our Appalachian belt, for example, 

 in Pennsylvania. Its geological representation is familiar enough, with its 

 bands of many colors disposed in intricate zigzags. The representation of 

 its orogenic disturbances would involve a greater share of doubt than in the 

 ordinary geological coloring, although even that is at this day not free from 

 question in all parts. There might be a strip of dull, dark color along the 

 Archean to show that this part of the region had been deformed even in 

 Archean times (the future student may subdivide this belt of color) ; an- 

 other of lighter and clearer tint along the Cambrian to indicate, perhaps with 

 some uncertainty, that there had been a Cambro-Silurian disturbance, not 

 extending far northwestward. There might be a brighter, stronger, and 

 more widely spread color to include all the area of the Appalachian folds, 

 this being the chief feature of the map. Finally, there might be a faint over- 

 tint along the Triassic belts to mark the Jurassic monoclinal tilting ; and 

 this might extend to the northwest of the Triassic belts as far as the folds 

 exhibit a prevailing overthrust in the direction of the Jurassic push. 



Then the representation of the date of the topographic forms might be 

 attempted. Its most striking feature would be the late date indicated by the 

 colors ; for while most of our rocks are Paleozoic, the dates of our forms are 



(582) 



