1890 .] 
583 
[Shaler. 
field, as by subsidence of the isthmian district of America permit- 
ting the tide to flow into the Pacific, this portion of the continent 
would be likely to enter upon a period of aridity. A yet more 
striking instance of important climatal change is indicated by the 
presence of extensive salt deposits in Eastern Louisiana immediately 
contiguous to the shores of the Gulf of Mexico. The precise age 
of this southern salt deposit is not well determined. It appears, 
however, probable that it was formed in comparatively recent geo- 
logical time. It should be observed that this field is now the seat 
of the most considerable rainfall which occurs in the eastern half 
of North America, north of the 25th parallel. In any condition of 
the climatic equations it seems impossible to suppose that rock 
salts could be accumulated in this portion of that shore of the Gulf 
of Mexico while the Gulf Stream had free access to its area. The 
geographic and geological position of this formation appears to 
exclude the supposition that it was deposited in a dead sea. The 
general character of the strata and all the other circumstances of 
its existence, seem to indicate that it is a coast-line deposit. It 
therefore becomes necessary to suppose that it was accumulated 
along an old seashore, and we are justified in arranging our sup- 
positions as to the conditions of the sea in this region in such 
manner as will account for the occurrence of the deposit. 
The salt deposits of England, Germany, and other portions of 
the earth afford similar evidence as to the existence of arid periods 
in various stages of the earth’s history in which they occur, re- 
gions which are now characterized by an abundant rainfall. 
From the facts already noticed it appears probable that in many 
instances the arid periods in which saliferous deposits were accu- 
mulated occurred in the geologic ages, when we have evidence of 
extensive precipitation in the same field, at a period not remote 
from the time of desiccation. Thus, in Europe, the Trias contains 
extensive salt deposits, though other portions of the section indi- 
cate by their composition an abundant precipitation and by their 
organic remains a tolerably warm climate. 
Although not yet in the form to warrant extended discussion, 
these facts clearly indicate sudden climatal revolutions at various 
stages in the past. 
Although I have spoken of ordinary salt deposits alone, it is 
evident that accumulations of other materials ordinarily precipitat- 
ed by evaporation, require attention. While indications of value 
may be derived from deposits such as the Stassfurt salts they are 
