Shaler.] 
582 
April 16, 
mulation took place in an interior or coastal basin. The character 
of the deposits and the nature of the fossils in contemporaneous 
strata will also afford evidence on this point. Still further the chem- 
ical nature of the bed may serve to show whether the precipitated 
materials were drawn from marine waters or from the substances 
brought in from the neighboring lands by tributary rivers. 
To utilize the data afforded by salt deposits in determining the 
existence of arid conditions in various countries in various ages, it 
has been found necessary to plat on a series of ’world maps the ac- 
cumulations which have been formed in the several divisions of geo- 
logical time, taking first those now forming and in succession those 
which w'ere accumulated at each stage in the geological develop- 
ment of the continents. Although this task is but begun, it is 
evident that the final result will afford some important conclusions 
concerning the ancient climates. Thus in the field about the great 
lakes of North America we have extensive accumulations of rock 
salts formed during the Silurian period. These accumulations of 
the Salina period were immediately preceded by deposits of the Ni- 
agara and were succeeded by those of Lower Helderberg age. The 
Niagara rocks afford abundant evidence showing that this region 
was covered by marine waters of considerable warmth. The abun- 
dant development of corals appears to indicate that the region was 
visited by a warm current coming from the south. Warm oceanic 
waters in a region of this high latitude must have led to an abun- 
dant precipitation on the neighboring more northerly shores. We 
are, therefore, entitled to believe that the land conditions in this 
district were those of a humid nature. On the other hand, the very 
thick deposits of rock salts which were accumulated immediately 
after the close of the Niagara beds appear to indicate that in a 
sudden manner the climatal conditions of this region were greatly 
changed, an arid state of the atmosphere replacing the humid con- 
ditions which had previously existed. Again, in passing to the 
Lower Helderberg, we find after the period of the Waterlime, evi- 
dence that the seas returned to something like their previous con- 
ditions of temperature. It appears legitimate to suppose that this 
alteration in climatal conditions may have been at least in part due 
to the presence or absence of the ancient gulf stream in the waters 
of this vicinity. Such a stream, flowing against the southern bor- 
ders of the Laurentian land area, would, it appears to me, have 
inevitably produced a warm sea and a humid climate of the neigh- 
boring shore. If the current were for a time withdrawn from this 
