136 The American Geologist. September, i896 
SiO., 98.10 per cent . 
AI263 0.24 " '• 
Ye.p.^ and FeO 0.27 " " 
Cab 0.184 " " 
Na.p 0.23 " " 
The organic matter is not over .008%. Professor Seamon 
suggests a different origin for this substance, viz. that it is 
produced by the exhaustion of the so-called " tallow clays" of 
this region. These clays were described by professor Seamon 
in an article in the American Journal of Science of January, 
1890, under the title "Zinciferous Clays of S. E. Missouri 
and a Theory as to the Growth of the Calamine of that Sec- 
tion." In connection with these beds Mr. Seamon has found 
decomposition products of a siliceous character greatly re- 
sembling the tripoli beds. The writer has been able to make 
a comparative microscopic examination of the various sub- 
stances referred to through the kindness of director Seamon. 
The tripoli itself is a granular aggregate with the average 
grain less than .01 mm. in diameter, though there is an ad- 
mixture of larger particles. The grains are irregular in form 
but are apparently crystalline rather tlian amorphous (in the 
absence of polariser this may be left an open question). A 
specimen of the decomposed chert, such as would be essential- 
ly identical with the tripoli upon the hypothesis advanced by 
Mr. Hovey, has a very different texture and seems to be a very 
unevenly sorted amorphous powder. While not denying that 
by a sorting under water this decomposed chert might give 
rise to beds like those in question, it yet is not easy to see how 
such deposits could occur under the conditions here prevail- 
ing. 
On the other hand, a specimen of exhausted tallow clay 
taken from one of these beds, under the microscope, is almost 
indistinguishable from the tripoli. Passing now to the bed 
of so-called tripoli in Socorro count}'', New Mexico, we have a 
deposit of a very different nature and one for which we must 
apparently seek a very different origin. This deposit occurs 
on the east side of the Rio Grande, nearly opposite the town 
of San Antonio. The immediate valley of the river, varying 
in width from six to twelve miles, is filled with stratified flu- 
vial deposits of sand, clay and gravel and the materials are, 
of course, such as may be derived from the adjacent forma- 
