254 Remarks on American Rock Formations. 
three bearing verniers, the fourth the clamping and tangent 
screw. k,k, is the outer alhidade, having also four arms, 
three bearing verniers, the fourth a clamping and tangent 
screw, for slow motion to the circle, the fourth is extended 
downward to a projecting piece, m, m, from the pillars, e, é, 
and furnished with a clamping and tangent screw, for the 
adjustment of the level, 1, fixed to the outer alhidade. On 
the upper horizontal alhidade is placed a compass, n, n, for 
the magnetic bearings, and two small levels, 0, 0, for adjust- 
ing the instrument more readily. p, p, is a larger level for 
the adjustment of the axis of the telescope, which passes 
through the arms of the vertical circle and its alhidades, and 
rests on the axis of the telescope, as shown in fig. 4; it Is 
removed after adjustment, An illumination is effected 
through one of the conical arms, g, g, by means of a small 
lamp, r, fastened to a support, s, s, that is attached to the 
three legged stand, by which it is detached entirely from the 
instrument, and of course can have no influence on its ad- 
justments, 
Art. VIIL— Remarks on the characters and classification of 
certain American Rock Formations; by Larpner Va- 
nuxem, late Professor of Chemistry, &c. in the College of 
South Carolina, in a letter to Professor Cleaveland.* 
In American Geology, there are, in my opinion, many al- 
terations to be made, and which would have long since been 
made, if observers, of different schools, had examined the 
regions, to which my assertions have reference. The allu- 
vial of Mr. Maclure, (as I made known in a paper left with 
the Academy of Nat. Sci. of Phil.) contains not only well char- 
acterized alluvion, but products of the tertiary and secondary 
classes. Littoral shells, similar to those of the English and Pa- 
ris basins, and pelagic shells, similar to those of the chalk de- 
position or latest secondary, abound in it. These two kinds of 
shells are not mixed with each other; they occur in different 
matter, and, in the southern states particularly, are 
at different levels. The incoherency or earthiness of the 
‘Spade 
_*Towhom the communication was originally made, in reference to the 
Reiroming new edition of his Mineralogy, and it is now published by the 
consent of the author and of his correspondent. 
