294 W. J. McGee—Origin and Hade of Normal Faults. 
would be effected in the ratio above given by subtracting 
enough calcium to make the ratio Ca: Al as 1:2 and an equiv- 
alent amount of fluorine. The atomic values then become: 
Al 0-809 2:00 
Ca 0°404 1-00 
F 1°656 ) 9. 7-91 
HO 1°539 ts i 
which agree quite as well for the formula CaAl,(F, OH), as 
those obtained by Brandl. 
If instead of the mean of all the analyses, the figures of II 
alone are taken for calculations similar to the above, the result 
is the same, even a little more closely approximating to the 
ratio 2:1:8. 
_ The observation made by Brandl, that below 260° C. no loss 
in weight occurs, was found to apply here, provided the expo- 
sure to this degree of temperature is short. If continued for 
many hours a slight but sensible loss is observed. 
Denver, Colorado, June, 1883. 
Art. XXXI—On the Origin and Hade of Normal Faults; by 
W. J. McGur.* 
sidered. 
Let contiguous areas of a homogeneous rigid tract resting on 
a mobile sub-stratum (which may be either the gaseous, fluid or 
a 
differential radial strain culmin- 
ating in the plane of coincidence 
of the unstable areas. 
* Read before the Iowa Academy of Science, May 31st, 1883. 
Os 
