464 C. SCHUCHERT PALEOGEOGRAPHY OF XORTH AMERICA 



obscure, but during the late Paleozoic, early Mesozoic, and the Tertiary 

 it was subject to much volcanic activity. The Xicola formation of Trias- 

 sic time alone has a thickness of 13,590 feet, nine-tenths of which are of 

 volcanic origin. 



o J 



Paleozoic Lands, or Positive Eleaeexts 

 (See map, plate 49) 



James D. Dana was apparently the first to point out the permanency of 

 oceanic and continental areas. From the standpoint of a Laplacian 74 he 

 stated the following in his paper "On the volcanoes of the moon/ 7 which 

 appeared in 1846 : 75 



"On our globe the continents have to a very great extent been long free from 

 volcanic action. A glance at a map of Asia and America will make this ap- 

 parent. It is usual to attribute this almost total absence of volcanoes from the 

 interior of the continents to the absence of the sea ; but it is fatal to this popu- 

 lar hypothesis, that the same freedom from volcanoes existed in the Silurian 

 period, when these very continents were mostly under salt water, a fact to 

 which the widespread Silurian rocks of America and Russia testify. Over the 

 oceans, on the contrary, all the islands excepting the coral, are igneous — and 

 the coral may rest as we have reason to believe on an igneous base. 



"It is therefore a just conclusion that the areas of the surface constituting 

 the continents were first free from eruptive fires. These portions cooled first, 

 and consequently the contraction in progress affected most the other parts. 

 The great depressions occupied by the oceans thus began ; and for a long period 

 afterward, continued deepening by slow, though it may have been unequal, 

 progress. This may be deemed a mere hypothesis ; if so, it is not as groundless 

 as the common assumption that the oceans may have once been dry land, a 

 view often the basis of geological reasoning. 



"Before the depression of the oceanic part of our globe had made much 

 progress, the depth would be too shallow to contain the seas, and consequently 

 the whole land would be under water. Is it not a fact that in the early Silu- 

 rian epoch nearly every part of the globe was beneath the ocean? [What he 

 has observed here is the Trenton inundation ; it is the largest to which North 

 America has been subjected.] . . . The depth of water over the continental 

 portions would be very various : but those parts which now abound in the 

 relics of marine life, were probably comparatively shallow, as amount of press- 

 ure, light, and dissolved air, are the principal circumstances influencing the 



distribution of animals in depth Here then we see reason for what 



has been considered a most improbable supposition, the existence of an im- 

 mense area covered in most parts by shallow seas and so fitted for marine life. 



74 According to the planetesimal hypothesis of Chamberlin and Salisbury, the perma- 

 nency of oceanic and continental areas, is also held, but the method leading to the origin 

 of the oceans is different from those described by Dana. (See their Geology, vol. II. 

 1906, pp. 84-88, 106-111.) 



73 Dana : American Journal of Science, vol. 2. 1S46, pp. 353-355. 





