Crosby.] 
138 
[May 21, 
proportion of rock-flour is a good indication, but not positive proof, 
of a glacial origin, and the application of this test to the older for- 
mations should be reenforced by entirely independent evidence. 
SAI/r IN THE DRIFT. 
In the eighth annual report of the United States Geological Sur- 
vey, for 1886-87, Prof. N. S. Shaler announces (p. 127) the dis- 
covery “that a certain quantity of sea-water remains imprisoned in 
some of our stratified clays formed at the close of the glacial period 
but now lying at various heights above the sea level.” The details 
of Professor Shaler’ s investigation are still unpublished, and this 
reference to it is made simply to render more apparent the geolog- 
ical interest and application of certain results which have be4n 
reached in a very thorough and systematic chemical and sanitary 
examination of the public water supplies of Massachusetts made 
during the last four years by Dr. T. M. Drown and Mrs. E. H. Rich- 
ards in the Sanitary Laboratory of the Massachusetts Institute of 
Technology and under the auspices of the State Board of Health. 
The chlorine, which exists of course in the form of sodium chloride 
or common salt, is regarded as affording the most reliable indica- 
tion of the contamination of the water supplies by drainage. It 
has been found, however, that all the natural waters of the State, 
and even the purest rain waters, contain appreciable amounts of 
chlorine. It became necessary, therefore, in order to give their 
results sanitary value, to determine for each locality what they 
have called the normal chlorine, /. e., the amount of chlorine found 
in the purest obtainable water and attributable entirely to the nat- 
ural conditions. 
In a region of crystalline rocks, like Massachusetts, it is in the 
highest degree improbable that a detectable trace of salt should find 
its way into the natural waters — the ponds, lakes, streams and 
wells — from any geological source, unless, indeed, it came from 
the drift deposits ; and in this case we should be obliged to adopt 
Professor Shaler’s suggestion that the salt represents sea-water which 
became incorporated with the drift at the time of its deposition or 
during some postglacial subsidence of the land. But, strangely 
enough, the normal chlorine is found in all altitudes and its varia- 
tions are, apparently, quite independent of the geological condi- 
tions. It was soon observed, however, that the normal chlorine is 
highest in the eastern part of the State, and this suggested the 
construction of a chlorine map, which shows that almost without 
