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determine the hardness of the mineral, would show as to 
whether the grains composing it, were Siliceous or Fels- 
pathic. At any rate ninety-nine out of a hundred of all 
existing Sandstones examined as yet, were Siliceous, and one 
of the difficulties which presents itself in the formation of 
Sandstones in which the grains shall consist of fragments of 
Felspathic minerals, is, that such minerals are extremely 
prone to decomposition by the action of moisture, and the 
atmosphere; the alkalies being dissolved out and alumina 
deposited in the form of clay. 
Dr. P. SCHWEITZER remarked that although the error 
which Prof. Seely pointed out, might appear, if mineral 
analysis were made by incompetent chemists, yet it was well 
known that careful observers, when examining Silicates, de- 
composed them by some means, as by the use of Hydro- 
fluoric acid, in such a way that the Silica was determined 
directly and not indirectly and by loss. 
Pror. T. EGnuEston exhibited a very large crystal of 
Magnetite, from Essex County, New Jersey, which, upon ex- 
amination, was found to present some features of considerable 
interest. It is a very composite Octohedron, showing no less 
than four distinct crystals. It is flattened parallel to two of 
its diagonally opposite faces, so that the crystal is found to 
be three inches long, by two inches wide. The appearances 
thus presented are so deceptive, that this form of crystal has 
been called ‘‘ Magnetite pseudomorph after Calcite ;” there- 
fore, and to correct this error which some mineralogists have 
fallen into, this specimen was exhibited. 
