THE ALEUTIAN ISLANDS 199 



Tertiary folded system of Europe and to the peripheral system of Eu- 

 rasia. 



Since there is no ground for separating Europe from Asia as dynamic 

 units in the Tertiary deformation, they should be taken together as one 

 unit, and the Tertiary ranges of the entire peripheral belt should be re- 

 garded as the product of southward creep of the entire crustal sheet of 

 Eurasia. But there is still a little more to be added even to this great 

 unit. 



THE ARC OF THE ALEUTIAN ISLANDS 



Suess regards the arc of the Aleutian Islands as an independent line, 

 not to be counted with the island arcs of eastern Asia, and he counts it as 

 a distinct and separate element in enumerating the boundaries of the 

 Pacific. This is only partly justified, for this arc is, in fact, distinctly 

 Asiatic in all its characteristics and affinities. The Asiatic character of 

 this arc is strongly shown in its pronounced curvature and its grand 

 sweep. It is, indeed, the most perfect of all the island arcs, and, like 

 the arcs of the Kuriles and north Japan, has a long, narrow ocean deep 

 close in front of it. Its backland belongs about two-thirds to Asia and 

 one-third to North America. The mountain chain which forms the arc 

 rises from the sea as it approaches Alaska, and continues in the same 

 curve to the highest part of the Alaskan Mountains. 



While the Aleutian arc is a perfect type of the Asiatic arcs, no such 

 arc occurs in either of the American continents. Probably Suess re- 

 garded it as independent, because it is so situated that it could not be 

 related to his Siberian vertex in the way that he finds the other island 

 arcs to be; but this is not believed to be an essential distinction. That 

 part of Asia which forms the backland of the western part of this arc is 

 just as much a part of the great crustal sheet of Eurasia as any other 

 part, and the distinctly Asiatic type of the arc shows that whatever spe- 

 cial conditions determined the peculiarities of the other island arcs af- 

 fected the backland of the Aleutian arc in the same way. 



A^Hien the Asiatic character of the Aleutian arc is recognized it becomes 

 at once apparent that the crustal sheet of Eurasia is not limited on the 

 east by a line through Bering Strait dividing this arc and its backland, 

 but includes the whole arc and the whole of its backland. Thus, Asiatic 

 character is carried eastward to the heart of the Alaskan Mountains, 

 where the curve of the Aleutian arc meets the Cordilleran ranges of 

 North America in a sharp angle. This meeting point falls near the 

 148th meridian of west longitude, and when we consider continental 

 boundaries with reference to Tertiary diastrophism, we must include in 



