423 
[General Meeting. 
since its elevation by numerous valleys. The same peneplain de- 
termines the crest lines of Kittatinny mountain and of the many 
similar ridges of Pennsylvania on the west, and of the Triassic 
trap ridges on the east. It may be seen descending below sea-level 
where the Cretaceous beds lie on it below New Brunswick. Most 
of the streams that have opened valleys in the Highland portion of 
the peneplain are simply revivals of old streams of a .11 earlier geo- 
graphic cycle ; but in the Triassic area, most of the streams are de- 
scended from courses superimposed upon the present rocks by the 
cover of Cretaceous strata that once stretched across them : and 
many of the streams appear to have been much affected by adjust- 
ments following upon their discovery of the trap ridges across 
which their initial courses had been unwittingly chosen. The el- 
evation of the ancient peneplain occurred so long ago that its 
weaker rocks have been again baselevelled ; and thus we find ex- 
planation for the even interstream surface of the Central plain. 
Another elevation of the land allowed the opening of shallow val- 
leys in the plain, and in this shape we find it. The moderate dis- 
tortion and numerous oscillations that accompanied the latest cycle 
of change still need much study, particularly with reference to 
their effect on river courses. As the preparation of good topographic 
maps is continued over the other states of our Atlantic slope, there 
will be opportunity to test many of the statements made here, for 
the sequence of changes we have considered is not local but wide- 
spread. 
General Meeting, December 4, 1889. 
The President, Prof. F. W. Putnam, in the chair. 
Dr. R. T. Jackson read a paper on “ Certain Points in the De- 
velopment of the Shell of the Mollusca, with Especial Reference 
to the Pelecypoda.” 
Dr. J. Walter Fewkes called attention to excavations made in 
rocks at Grand Manan, New Brunswick, by our common sea-ur- 
chin (see American Naturalist, January, 1890). 
