246 Messrs. Jackson and Alger on the 
its frowning summit, from which large masses of rock, de- 
tached by the frosts, are almost continually falling. 
Near this point, under the protection of an arch of columnar 
trap, adeep cavity was discovered in the amygdaloid, which, 
having a narrow aperture, expanded internally to the diameter of 
six feet, in every direction. The mouth of this cavern being . 
enlarged, so as to admit of examination, its walls were found to 
be thickly encrusted with laumonite in a remarkably fine state of 
preservation. Specimens were easily detached by the hand, and 
were found to consist of successive layers of radiating crystals, 
which, in the centre of the mass, were of a fine flesh-red color. 
The external surface of this crust, and the interior of cavities 
which frequently occur, were richly studded with transparent and 
colorless crystals, of great perfection and beauty. They are in 
the form of the oblique rhombic prism, terminated by a rhombic 
plane passing from one of the acute solid angles to the other, and 
almost constantly replaced on the acute solid angles by a single 
triangular plane resting on the acute lateral edges; these sec- 
ondary planes are always small, and never obscure the primary 
form of the crystal. The cavities, in the laumonite, are often 
filled with water, which serves to prevent the efflorescence of the 
crystals, which are thus preserved in an unaltered state. The 
surface of this mineral is frequently enriched with crystals of 
calcareous spar, exhibiting the forms of the rhomboid more 
obtuse than the primary, and the scalene triangular planed dode- 
cahedron. Large and perfect crystals of apophyllite, in the form 
of the square prism, generally replaced on the solid angles by 
single triangular planes, which are in various degrees of advance- 
ment, sometimes almost concealing the primary form, are found 
at this place. This mineral agrees perfectly with specimens in 
