POSITIVE ELEMENTS 465 



"If we follow the progress of the land, we find that with each great epoch 

 there has been a retiring of the sea. . . . Subsequently, the progress on the 

 whole was giving increased extent and height to the land and diminishing the 

 area of the waters. Instead therefore of a bodily lifting of the continents to 

 produce the apparent elevation, it may actually have been a retreating of the 

 waters through the sinking of the ocean's bottom. The process however has 

 not been a continuous one: for during each epoch . . . there have been 

 subsidences as well as seeming and actual elevations, and various oscillations 

 of the continental surface, from subaerial to submarine and the reverse. 



"And why should not the ocean's bottom subside, as well as the land? [He 

 states that there are 200 subsiding islands in the present Pacific (Dana, 1847: 

 94. ) ] What has given the continental portions of our globe their elevation, as 

 compared with other parts, if not the unequal contraction of the whole?" 



"Ruptures, elevations, foldings and contortions of strata have been produced 

 in the course of contraction. The greater subsidence of the oceanic parts would 

 necessarily occasion that lateral pressure required for the rise and various 

 foldings of the Alleghanies and like regions" (1846: 352-355). 



"If then, the typical form of a continent is a trough or basin, the oceanic 

 border being raised into mountains; if these borders are so turned as to face 

 the widest range of ocean ; if the height of these border mountains and the ex- 

 tent of the igneous action along them is directly proportioned to the size of the 

 oceans, — the Pacific, accordingly, being girt with great volcanoes and lofty 

 mountains, while the narrower Atlantic is bounded by smaller heights and but 

 few volcanoes ; if, moreover, volcanoes characterize the islands of mid-ocean 

 and not the interior of the continents: what is the legitimate inference? 



"Most plainly, that the extent and positions of the oceanic depressions have 

 some way determined, in a great degree, the features of the land ; that the same 

 cause which originated the one, impressed peculiarities on the other ; that the 

 two had a parallel history through past time — the oceanic depressions tending 

 downward, the continents upward; in other words, that they have both been 

 in progress with mutual reaction from the beginning of the earth's refrigera- 

 tion. The continents have always been the more elevated land of the crust, 

 and the oceanic basins always basins, or the more depressed land." 78 



We have seen how the lands or positive elements originated, and in the 

 language of Willis 77 these are characterized as follows : 



"The geologic characteristics of a positive element are deep denudation, an 

 absence of sediments of critical periods, and the corresponding prolonged dura- 

 tion of the sum of unconformities. . . . They may have been depressed 

 relatively to adjacent areas to some extent, but the algebraic sum of vertical 

 movements has been upward, and has been positive as compared with other 

 parts of the continent and the neighboring ocean bottoms. Whether they be 

 regarded as horsts or protrusions resulting from radial elongation, their move- 

 ment is positive, and they may fitly be called positive elements." 



Late in Proterozoic time, or just previous to the introduction of the 



76 Dana : American Journal of Science, vol. 22, 1856, pp. 338-339. 

 " Willis : Bull. Geological Society of America, vol. 18, 1907, p. 393. 



