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IOWA ACADEMY OF SCIENCE Voi.. XXVIII, 1921 
smooth lowland to the eastward its summital plain is about 4000 
feet above tide. On the northeast the uplift rises abruptly out of 
a widespread desert lying below sea-level, the Mexican extension 
of the Imperial Valley, recently reclaimed extensively to garden 
purposes through irrigation. 
The central massif is a granitic type of rock not so very unlike 
that of the Sierra Nevada farther north. This granitic core is con- 
spicuously jointed after the usual manner of large granite mass- 
es, but the joint planes differ from the common occurrences of 
this kind of structure in that the faces of the ruptures are separat- 
ed widely and the intervening spaces are filled with white, aphan- 
itic, or quartz-like material, which produces a wonderful highly 
contrasted network. It is veining on a colossal scale, the joint 
plates being two to six feet across. 
Toward the north end of the mountain range the titanic crazing 
is displayed in superb sections 1500 feet high in the famous Caris- 
so Gorge near the United States boundary. The characteristic 
effects are well represented in the accompanying photographic 
view (figure 5). 
