No. 50.] 
323 
The extent of surface which is occupied by the Potsdam sandstone 
may be defined by drawing a hne from, or near Carthage to Perch lake, 
and thence to the Oxbow, by Theresa falls. There are, however, on 
the east side of this line many broken masses of sandstone, but none 
which are continuous for any considerable extent. Another portion of 
this rock appears at French creek ; it constitutes a part of Grindstone 
island. This rock is necessarily characterized by its position and mi- 
neral structure ; as the only fossil which abounds in it is the lingula 
ovata, and large portions of it are entirely destitute even of this. But 
it may be considered as a characteristic fossil wherever it appears. It 
is found in the granular and friable masses one mile east of French 
creek on the road to Depeauville. This fossil, too, is well known to 
occur at Keesville ; it is therefore widely distributed. It is an ex- 
tremely thin, small shell, and hence easily escapes notice unless it is 
particularly sought for. This circumstance indicates a great scarcity 
of lime in the waters from which the rock was deposited, or a great 
scarcity of food ; for we find that in our fresh water lakes those unios 
or other genera of animals, if the waters are free from lime, have always 
their delicate shells ; whereas, in those which are highly charged with 
lime they are thick and heavy. 
In proceeding upwards we next meet with a thin mass, composed of 
about equal proportions of carbonate of lime and sandstone ; and which 
is filled with marine plants. It has received the name of fucoidal 
mass. It merits this distinction, for it uniformly occupies this posi- 
tion ; it is evident too, that an important change had commenced in the 
nature of the earthy materials. Previously, no fossil of this kind had 
appeared ; the waters of the ocean had been too much troubled to per- 
mit the growth of sea weeds on its bottom ; but at this period an abun 
dance suddenly appears in the waters, showing thereby a quiet, settled 
condition ; and moreover, an accession of matter from a new source, 
viz : carbonate of lime. 
This mass varies from 2 to 10 feet in thickness. It appears at the 
summit of the hill one mile east of French creek. It was uncovered 
on repairing the road at this place. The lingula ovata may be found 
just below in the friable sandstone, lying by the roadside. Though the 
fucoidal mass is very thin, comparatively, still it is a very constant 
rock, and is rarely absent ; thus it is found on Lake Champlain at nu- 
merous places, bearing precisely the same fossil. 
