50 General Notes. ; (January, 
ing the Genesee Shale from the Hamilton Shales a thick band of lime- 
stone — the Tully Limestone — rises from the water and after an undu- 
lating course of several miles passes off at the surface. Directly be- 
neath this is the Moscow Shale, a dark, laminated mud-rock easily 
disintegrated by water, the Encrinal Limestone, — a very thin bed, — 
and the Ludlowville Shale. The Tully Limestone forms a rocky table 
upon which the streams often flow for a considerable distance, the dip 
of the bed frequently being so slight as to present nearly a level surface. 
As they approach the lake the water flows over the hard rocky table 
and cuts its way through the softer deposits. These are easily disin- 
tegrated by the combined action of frost and water, and are washed away, 
forming caverns below the limestone, which after a time breaks off, leav- 
ing large masses in the bed of the stream. This is particularly well 
exemplified in the glens about Ludlowville (eight miles north of Ithaca) 
and at Shurgur’s Glen, near the lake shore. Both of these localities are 
much frequented by collectors. Spirifera granulifera, S. medialis, S. 
mucronata, and Athyris spiriferoides are found there in great abundance, 
also Phacops bufo and other trilobites, many species of lamellibranchs, and 
a number of cephalopods. This formation, known to geologists as the 
Hamilton Group, including the Tully Limestone, the Moscow Shale, the 
Encrinal Limestone, and the Ludlowville Shale, continues for many miles 
along the lake shore. From an economic stand-point the Tully Lime- 
stone only is important, being valuable for lime and building purposes. 
The only minerals found are calcite, in small quantities, and iron pyrites. 
We turn now to the superficial deposits and water-courses. At Ithaca 
there are two distinct types of river or creek valleys —the one with 
rounded and well-worn sides, the other bordered by precipitous walls of 
rock. ‘To the latter class belong Cascadilla and Fall creeks, which flow 
into the Ithaca plain from the east. Their valleys are true valleys of 
erosion, having been formed since the withdrawal of the vast ice-sheet 
which swept over this portion of North America in quaternary time. 
With the exception of Six Mile Creek Valley and that of Cayuga Inlet, 
which open into the lake basin from the southeast and south respectively, 
all the streams of this immediate vicinity flow through deep cuts or 
cafions, in which they descend by numerous cascades and water-falls to 
the lake. As their valleys are mere chasms, they make no appreciable 
change in the general contour of the land. With valleys of the first 
type, however, the effect is of an entirely different character. They are 
distinctly marked. ‘Their longer slope and greater width make a prom- 
inent feature in the topography of Ithaca. Noting in addition the depth 
at which the water flows, and the small number of cascades and water- 
falls, the conclusion is at once reached that these valleys have been acted 
upon by some agency not now in operation. We can observe changes 
going on in Fall and Cascadilla creeks ; we can easily understand how 
their deep, rocky cafions could be formed and are still being formed by 
