68 
Bulletin Wisconsin yaturnJ History Society. [Vol. 5, Xo. 2. 
historj' of the later elephants of the United States that have only 
recently disappeared. After the lecture President Teller thanked 
Professor "Williston for his kindness in coming to Milwaukee to lecture, 
and expressed the great appreciation of the Natural History Society. 
The meeting then adjourned. 
Milwaukee, March 21, 1907. 
Meeting of the combined sections. 
In the absence of President Teller, Vice-President Ward presided. 
Mr. George W. Colles spoke on the Classification and Origin of the 
Micas. 
The term mica embraces a group of minerals which have certain 
marked chemical and physical characteristics, the principal of which 
is their cleavabilitj^ into thin flexible plates. They are all compound 
hydrous silicates of aluminum and another base of a highly complex 
character. As is usual in complex minerals of this sort, the chemical 
composition is not fixed and invariable as in quartz, calcite and gypsum, 
and consequently to find a rational chemical formula which would 
embrace all the mica minerals was a very difficult matter. It was 
accomplished, however, by Mr. F, W. Clarke, who has given much 
study to the question of chemical formulas for minerals. 
According to the old standard classification, which seems to have 
been followed hy everyone who has written on the subject until the 
publication of Colles' work, all mica was divided into three sorts, 
termed muscovite, phlogopite and biotit^. There is neither rhyme 
nor reason in such a classification as this. While the species known 
as muscovite and phlogopite belong in two separate groups, the term 
biotite either was applied to two diflierent types or else it represented 
a type of which phlogopite was a mere varietj-. There are really 
two different main groups into which all the numerous species and 
varieties of mica that have been described naturally fall. Chemically, 
these are distinguished as monad and dyad types, that is, types in 
which the positive radicals are monads or dyads, respectively. Because 
of this difference in chemical composition the two classes have been 
called perissad and artiad micas, respectively, from the Greek words 
for "odd" and "even." In the perissad group the principal monad 
radicals are H, K, Li, Al(OH)o, AlFo, AlO, Fe'", and in case of the 
semi-mica paragonite, Xa. The principal dyad bases are ^Ig, Fe", 
AlOH and AlF. 
