Editorial Comment. 249 
contained material for a chronological sub-division of Pleistocene 
-deposits. 
SESSION OK AX'G. 28, 1891, 11 :40 a. m. 
Prof. J, Le Conte, Vice-President, in the chair. 
Announcements by the General Secretarj% Mr. S. F. Emmons, relative 
to the minutes of yesterday and to various excursions. 
Announcement by major Powell in regard to the essays on correlation 
to be published as bulletins of the U. S. Geological Survey. 
The President then announced as the subject for discussion, the Cor- 
relation of Geological Formations. 
Mr. Gilbert opened this discussion by presenting a general classifica- 
tion of methods of correlation. 
Strata are locally classified by superposition in chronologic sequences. 
Geologic correlation is the chronology of beds not in visible sequence. 
For convenience in discussion, methods of correlation are classed in ten 
groups, of which six are physical and four biotic. 
PHYSICAL METHODS OF CORRELATION. 
1. Through \isible continuity. The outcrop of a bed is traced from 
point to point and the different parts are thus correlated one with 
another. 
2. Strata are correlated on account of lithologic similarity. This 
method, once widely prevalent, is used where the distances are small. 
3. Correlation by the similarity of lithologic sequence has great and im- 
portant use where the localities compared fall within the same geologic 
province, but it is not safely used in passing from province to province. 
4. Physical breaks or unconformities, have a limited use, especially 
in conjunction with other methods. The practice of employing them in 
the case of localities wide apart is viewed with suspicion. 
5. Deposits are also correlated with their simultaneous relations to 
some physical event; for example, a beach with the lake beds it encircles; 
a base level plane with a contiguous subaqueous deposit ; and alluvial, 
littoral, and subaqueous deposits standing in proper topographic relation. 
In the Pleistocene, glacial deposits are widely correlated with reference 
to a climatic episode assumed to arise from some general cause. 
Deposits are correlated through comparison of changes they have ex- 
perienced from Geologic processes supposed to be continuous. Newer 
und older drift deposits in different regions are correlated according to 
the relative extent of weathering and erosion; induration and metamor- 
phism afford presumptive evidence of age, but yield to evidence of other 
character. Metamorphism holds prominent place in the correlation of 
Pre-Cambrian rocks where most methods are inapplicable. 
Tliese physical methods are (lualified by the geographic distribution 
of Geologic processes of change and of geologic climates. 
BIOTIC METHODS OF CORRELATION. 
7. A newly-discovered fauna or flora is compared with a standard 
series of faunas and Horas by means of the species it holds in common 
with them severally. 
