DIASTROPHISM AS BASIS OF CORRELATION 691 
base-level, may easily be added, and perhaps on quite as firm or even 
firmer physical grounds. If we add the biological element the case 
is immeasurably strengthened, for correlation by cosmopolitan faunas, 
the very best of faunas for the purpose, is added to the physical 
correlation. Migration at the climax of base-leveling and sea- 
transgression is freer and more prompt than at other times. Corre- 
lation to the foot, as by an unconformity, may not be practicable, but 
the precision of correlation by unconformities has more apparent 
than real value, for the different parts of the same unconformity vary 
much in time. All distant correlations involve some measure of 
inexactness, and the more frankly it is made obvious, the less its 
liability to mislead. 
Correlation by general diastrophic movements takes cognizance 
of four stages: (x) the stages of climacteric base-leveling and sea- 
transgression, (2) the stages of retreat which are the first stages of 
diastrophic movement after the quiescent period, (3) the stages of 
climacteric diastrophism and of greatest sea-retreat, and (4) the 
stages of early quiescence, progressive degradation, and sea-advance. 
(1) The characteristics of the climacteric stage of base-leveling 
and sea-transgression need little further characterization here, for the 
function of base-levels is known to all American geologists and the 
function of great sea-transgression to every stratigrapher and paleon- 
tologist. We have in base-leveling conjoined with sea-transgression, 
just that combination of agencies which is competent to develop the 
broad epicontinental seas of nearly uniform depth requisite for an 
expansional evolution of shallow-water life. At the same time, it 
furnishes broad pathways around and across the continental surfaces 
for wide migrations and the comminglings that lead to cosmopolitan 
faunas of the shallow-water type. 
(2) The stages of initial diastrophism and sea-retreat find their 
criteria in the deposits that spring from an increased erosion of the 
deep soil-mantles accumulated in the base-level period, in the effects 
of increasing turbidity, in the lessening areas suitable for the shallow- 
water life, and in the limitation of migration. 
(3) The climacteric stages of diastrophism are marked by the 
stress of restrictional evolution among the shallow-water species; by 
increased clastic deposition in land basins, on low slopes, and on sea 
