AMERICAN GEOLOGY DECADE OF 1840 L849. 425 



form of a frothy scoria which must on either side, in part, return to 

 supply the place of the ascending current. On cooling, then, the 

 more basaltic portions would constitute this exterior descending part. 

 Thus, a feldspathic center and basaltic flanks would be the result of 

 one and the same process. This feldspathic center, further, by being 

 inclosed within a thick covering of rocks, would cool slowly, forming, 

 perhaps, disseminated crystals in the earthy base, or, if cooling suffi- 

 ciently slowly, a crystalline granular mass like granite or syenite. 

 He recognized the fact — 



that particular rocks have no necessary relation to time, excepting so far as time is 

 connected with a difference in the earth's temperature or climate and also in oceanic 

 or atmospheric pressure, for, if the elements are at hand, it requires only different 

 circumstances as regards pressure, heat, and slowness of cooling to form any igneous 

 rock the world contains. 



The date of the beginning of volcanic activity on the Hawaiian 

 Islands he placed as far back as the early Carboniferous or Silurian 

 epoch, and believed that: (1) There were as many separate rents in the 

 region as there are now islands; ('2) that each rent was widest in the 

 southeast portion; (3) that the southernmost rent was the largest, and 

 (4) that the order of extinction of the volcanoes was as follows: 



1. Kauai. 



2. Western Oahu. 



3. Western Maui (Mount Eeka). 



4. Eastern Oahu. 



5. Northwestern Hawaii (Mount Kea). 



6. Southeast Maui (Mount Kale-a-kala). 

 T. Southeast Hawaii (Mount Loa). 



From the general arrangement of the islands in the Pacific and the 

 study of the phenomena connected therewith the conclusion was reached 

 that the Hawaiian group originated in a series of rents or ruptures 

 seldom continued at the surface for a long distance, but frequently 

 advancing successively, one after the other, causing the resultant 

 islands to appear in the form of a curve rather than a straight line, 

 and. further, it was announced that: (1) While straight ranges are of 

 occasional occurrence, curved ranges are still more common; (2) that 

 curvature may arise either from a gradual change of trend in the sub- 

 ordinate parts or from the position of these parts in a series; (3) that 

 the same great chain may change its direction 60 c or more, and con- 

 sequently (4) the course of a mountain chain can be no evidence of its 

 age. In this it will be observed he differed radically from Elie de 

 Beaumont. 



Dana noted further that the Pacific islands were arranged mainly in 

 two systems of linear groups nearly at right angles with each other, 

 the linear groups being based on a series of ruptures instead of a 

 single uninterrupted fissure. The prevailing uniformity of trend of 



