Tlic Kezvecnaivaii in Miimcsota. — Elftman. 175 
push the wedge" with its thin end forwards through the field of 
view of the microscope. Note the colors as they rise in the 
scale, as the successively thicker portions of the wedge pass 
in view. The scale thus noted will be suited to the wedge 
employed and to the student using it at that stage of his ex- 
perience. The operation can be repeated with the nicols par- 
allel if desired. 
IV. Section and Plate, 
I have found it convenient in practice to distinguish the 
terms "section" and "plate" in the microscopic study of min- 
erals and rocks as follows: 
The term "section" is employed to indicate the entire mass 
of the rock or mineral that is carried by the glass slide used on 
the stage of the microscope. The term "plate" is introduced 
to designate a particular section or slice of mineral or other 
substance that forms a part of the rock or general mass car- 
ried by the glass slide. A "section" is composed of "plates." 
A rock "section" is usually made up of many mineral "plates" 
either held together by intercrystallization or by some cement- 
ing material which material in its turn lies in an irregular 
"plate" or in "plates." 
"Plate" is never the equivalent of "section," unless a single 
"plate" of one mineral forms the entire "section." 
THE GEOLOGY OF THE KEWEENAWAN AREA IN 
NORTHEASTERN MINNESOTA. II. 
By A. H. Et.ftman, Minneapolis. 
Part II. GP:0L0GY OF THK KEWEENAWAN .SERIE.S. 
Chapter I. ."^iratigraphv. 
/. Histoi'ical RcTtci^'. 
The Keweenawan rocks of northeastern Minnesota are 
distributed over an area of about 4,500 square miles, and con- 
siderable has been written concerning this area. In view of 
the great diversity of opinions expressed, and as much that 
has been written consists of details regarding the geographi- 
cal distribution of the different members of this series, and 
