i) Syren aa 
J. W. Powell's Survey of the Rocky Mountain Region. 355 
are, that they began very soon after the close of the lacustrine 
period, and they may have commenced still earlier. The erup- 
ive epoch was undoubtedly a long one. The individual flows 
are very numerous and represent all the great groups of eruptive 
rocks. In many cases the quantity of material extravasated is 
so great that the eruptions may we called massive, not 
however of such marvellous extent as asserted to have been 
ave ema- 
nated during recent or modern times from any existing volcanic 
and precision. so vast are the accumulations and so expansive 
are the sheets, and at the same time so numerous that whereso- 
ever they were emitted the earlier vents must have been buried 
by later deluges of lava, and even the more recent vents, except 
in the case of the latest basalts, have been swept away by slow . 
erosions in the long period which has elapsed since their activity 
was extinguished. There are, however, still remaining, distinct 
a certain sense uneonformity of the various eruptions, and 
greater irregularities in their bedding as compared with the 
¥. y 
ency took place after the close or during the decadence of the 
tean Province occurred. During its progre I 
must have taken place, and their later age 18 readily identifia- 
