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, e, 

 and furnished with a clamping and tangent screw, for the 

 adjustment of the level, ?, fixed to the outer alhidade. On 

 the upper horizontal alhidade is placed a compass, n, n, for 

 the magnetic bearings, and two small levels, o, o, 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. VIII. — Remarks on the characters and classification of 

 certain American Rock Formations; by Lardner 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 

 earthy matter, and, in the southern states particularly, are 

 at different levels. The incoherency or earthiness of the 



* To whom the communication was orioinally made, in reference to the 

 forthcoming new edition of liis Mineralogy, and it is now published by the 

 consent both of the author imd of his correspondent. 



