The Second Lake Alf/ujiqm'ji. — Tai/lor. 115 
between these points onl}' the northern element of change has 
been effective. The importance of this inferential extension 
of the Nipissing plane is apparent when we consider how 
greatl}'^ it extends the known area of subsequent deformation. 
The Isobases of Defurmafiou* We are now prepared to 
examine the map wliich accompanies this paper. The Nipis- 
sing beach is nearly everywhere very close to the present lake 
shore, and for this reason there would be "no advantage, in a 
map drawn on so small a scale as this, in trying to represent 
that beach as an independent feature. The coast lines of the 
lakes are drawn heavier along all the shores where the Nipis- 
sing beach has been abandoned than elsewhere. In a few 
places, as towards North Bay, the beach passes inland, and 
the former outlet to the Ottawa valley is shown. The heav}' 
broken line east of North Bay is a conjectural representation 
of the shores of the arm of the sea into which the outlet river 
emptied. Where the beach passes under the present level of 
the lakes, it is represented b^^ a heavy broken line outside the 
present coast line. At the west end of lake Superior the de- 
pression represented is below that lake and not below the 
Huron plane, which is about 20 feet lower. 
The straight lines drawn across the map from southeast to 
northwest are lines of approximately equal deformation. They 
are the "isobases" of Baron de Geer.f The lower line, AA, 
which passes from near BulFalo toward Duluth, is the node 
line or line of intersection between the Nipissing and the 
Huron-Michigan planes. Along this line the Nipissing plane 
is calculated to pass under the present level of the lakes toward 
the southwest. Each of the other lines parallel to AA passes 
through places which have undergone approximately equal 
*It is only fair lo state iu-rc that neitlier tlie field work nor the subsc- 
(inent \vriti"n<>- of thepai)(;rs descriptivt^ of it were in the sli,i,'htcst degree 
prejudiced in favor of the very uniform attitude and extension of the 
Nipissintj plane as here described. It was hoped at first tliat only a 
simple and uniform northward rise would be found. Hut when the jon-r 
rise from Duluth to North P.ay was discovered, that idea had to be •riven 
up, and the observations htiiiceforth were made without any preconcep- 
tion as to what the resiilts wore goi'ifj to be. The real extent and atti- 
tude of the Nipissini,' plane was not worked out until after the first four 
papers had been published and the fifth stMit U) the editor, when the 
history of lake Algonquin was taken up as a special study. 
f -On Pleistocene Changes of I^evel in eastern Nt)rth Am<'rica," by 
IJaron Gerard de Geer. Proc. l?oston Soc. Nat. Hist., vol. xxv, May IS. 
181I2, with map; Amekkan (Jeoiakust, .Tanuary, l.S!):{. 
