36 PHILOSOPHICAL SOCIETY OF WASHINGTON. 
253p MEETING. , May 10, 1884. 
The President in the Chair. 
Fifty-four members and guests present. 
Announcement was made of the election to membership of 
Messrs. JonN Murpocu, Romyn Hircucock, WiLiiam SMITH 
Yeates, GrorGE Perkins MERRILL, and FREDERIC PERKINS 
DEWEY. 
It was announced that a vacancy in the General Committee, 
occasioned by the resignation of Mr. J. J. Knox, had been filled 
by the election of Mr. F. W. CLarkeE. 
By invitation, Mr. G. H. Witiras, of Baltimore, Maryland, 
addressed the Society on 
THE METHODS OF MODERN PETROGRAPHY, 
first, defining the field of petrography, and second, discussing the 
methods of petrographic investigation. These methods dre: (1), 
chemical; (2), mechanical; (3), optical; (4), thermal. The chem- 
ical methods are quantitative and qualitative. The mechanical 
methods include the separation of the constituent minerals of rocks 
by precipitation in heavy solutions and by the use of electro-mag- 
nets. The optical methods include the preparation of thin sections, 
their examination by transmitted ordinary light, and their exam- 
ination by polarized light, for the determination of crystallographic 
system, pleochroism, and angles of extinction. The thermal methods 
are chiefly synthetic, consisting in the artificial production of min- 
eral aggregates for the purpose of determining the processes of their 
natural production. By the regulation of temperatures in fusion 
and refrigeration all varieties and all structures of basic rocks are 
reproduced. Acidic rocks have not been thus reproduced, and it 
is believed that great pressure is a condition of their genesis. 
Mr. Durrton spoke of the bearing of modern petrographic in- 
vestigations on some of the greater problems of geology. 
