Maack.] 192 _- [June s, 
nite, like the Cordilleras of South and of North America. He 
said further, that tertiary strata filled with shells exist on the 
Atlantic side of the Isthmus, as well as on the Pacific side, 
and stretch across the Isthmus. These paleontological facts 
prove that up to the Pliocene pered both oceans mingled 
their waters. 
Dr. T. M. Brewer called the attention of the Society to the 
remarkable abundance of the Bay-breasted Warbler, Den- ~ 
droica castanea, both in the vicinity of Boston and in south- 
ern Wisconsin, the present season. 
They were observed in large numbers by Mr. Wm. Brewster, Mr. 
H.W. Henshaw and others, and quite a number of specimens ob- 
tained. Mr. Brewster thinks he met with as many as fifty individ- 
uals in a single forenoon. They were present in considerable numbers 
during the 26th, 27th and 28th of May. All our earlier writers speak 
of this warbler as one of our rarest birds, and it has been only for a 
few years known to be abundant in the northern parts of Maine dur- 
ing the breeding season. 
In Wisconsin this warbler appears to have been equally abundant. 
Mr. Kumlien writes: “The D. castanea, which used to be a rare 
bird with us, except in the latter part of August and in September, 
when they return with their families, has this week been very 
abundant; almost any number could have been had.” 
It is an interesting problem to solve the question, why this bird, 
usually so very rare, almost unknown along this parallel of latitude, 
should this season be so very abundant, probably from the Atlantic 
to the Mississippi? The hypothesis of Mr. Brewster is at least a 
possible explanation, which is that this species ordinarily makes its 
northern migrations in long extended flights, in which it passes over 
large tracts of country without making any pause. It passes noth 
quite late; is one of the very last of the migrants, and passes on to 
its breeding places without remaining or ~pausing in the section. 
For some reason, not to be so easily explained, it has been induced to 
deviate from its usual routine, and has found it convenient to make 
a stopping place both wath us in southern New England and in Wis- 
consin. 
In partial confirmation of this theory Dr. Brewer read an extract 
from Mr. Salvin’s paper in the “ Ibis” for April, 1872. showing that 
