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initiated by earth movements (tectonic) is a matter worthy of careful 
investigation. Its general appearance would seem to indicate a strue- 
tural origin in comparatively recent time, with considerable modification 
by erosion. 
The Mountain Belt—Within the mountain belt the road winds 
about along the valleys and across low divides, giving here and there 
glimpses or longer views of certain prominent mountains, such as San 
Miguel, Jamul and San Isidro, which according to the work of Fair- 
banks are composed of igneous types of rocks (porphyries), more re- 
sistant to erosion than the granites or the later sediments. 
Wherever the rocks were exposed along our road, or where, in 
particular, an opportunity was given to get out and examine them more 
closely, they were of granite (in the general field sense) or related or 
genetically associated types. This held, with but few minor exceptions, 
alone our whole course across the mountain belt and into the desert. 
We may look upon the mountain belt that lies between the San Diego 
Coast and the Imperial Valley as essentially a great granite mass, show- 
ing at its surface only subordinate exposures of rocks of unrelated 
types. 
It has apparently been definitely established that granitic rocks of 
this character are products of fusion and are necessarily formed un- 
der a thick mantle of rock. If we picture the conditions at the time of 
the consolidation of the granites, we must imagine their upper surface 
to have been far below the earth’s surface, and to have been succeeded 
upwards by a thickness of metamorphosed rock—probably several thou- 
sand feet—and finally, perhaps, by rocks, extendine to the earth’s sur- 
face, whose composition and texture were unaffected by the heat or ma- 
terial emanations of the granitic magma or by the agencies which had 
developed the molten mass. The evidence suggests that this granitic 
mass underlay a mountain area at that time. though its form and extent 
were probably not the same as the mountain belt of the present time. 
The excursion gave a good opportunity to observe how completely the 
original covering of the granite has been eroded, not simply along cer- 
tain lines of deep stream cutting but over broad areas. In fact the orig- 
inal ‘‘roof’’? appears to have been wholly removed and with it a cer- 
tain thickness of the granite itself. 
This gives evidence of a very long period of erosion, one not simply 
long enough to eut a system of canyons or valleys thousands of feet 
deep, but one long enough to cut away the divides and remove in toto 
from some thousands of square miles, thousands of feet of the outer 
crust. And this necessarily long period of erosion means that the ter- 
ritory involved was an exposed land mass during much or all of the 
