858 J. W. Powell's Survey of the Rocky Mountain Region. 
ters, as the cycle proceeds. Thus at the commencement of the 
activity we have propylite and hornblendic andesite, which are 
closely assimilated to each other in their physical characteris- 
tics; at the middle stage we have trachytes and moderately 
basic dolerites, which are moderately separated, and at the close 
we have rhyolites and basalts, which stand at the opposite ends 
of the scal 
and to correspond with each relief map a stereogram in plaster 
18 constructed on the same scale, designed to exhibit such sur- 
[To be continued. ] 
