THE 



AMERICAN JOURNAL OF SCIENCE 



[THIRD SERIES.] 



Aet. XXX VII. — On the Origin of Normal Faults and of the 

 Structure of the Basin region ; by Joseph LeConte. 



I have already, in a previous paper (Am. Geo!., vol. iv, p. 38), 

 given reasons for thinking that the general structure of the 

 earth is that of a solid nucleus constituting nearly its whole 

 mass, a solid crust of inconsiderable comparative thickness, and 

 asubcrust liquid layer, either universal or over large areas, sepa- 

 rating the one from the other. In this paper I assume such a 

 general constitution. I assume also that the crust rests upon the 

 subcrust liquid as a floating body. We may well assume this 

 because, broken as we know the crust to be, if it were not so it 

 would long ago have sunk into the subcrust liquid. I have also, 

 in the previous article already alluded to, shown that this con- 

 dition of flotation would be the necessary result of the increas- 

 ing density of the earth as we go down. 1 now wish to apply 

 these two assumptions to the explanation of Normal Faults and 

 of the origin of the Structure of the Basin region. 



Crustrfissures and great faults. — Leaving aside the small 

 fractures called joints which affect rocks of all kinds and in all 

 places, the crust of the earth, as is well known, is everywhere 

 traversed by great fissures more or less parallel to one another 

 in the same region, often hundreds of miles in length, and pass- 

 ing entirely through the crust into the subcrust liquid beneath, 

 by which the crust is broken into great oblong crust-blocks many 

 miles in extent and through which the subcrust liquid is often 

 outpoured on the surface in the form of lava sheets. The walls 

 of such fissures do not remain in their original position but are 

 always slipped, one side being heaved and the other dropped. 



Am. Jour. Sci— Third Series, Vol. XXXVIII, No. 226.— October, 1889. 

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