268 Messrs. Jackson and Alger on the 
Before alluding to these minerals, we would observe that the 
compact trap forming the highest parts of this island is rarely in 
masses which may strictly be called columnar, although they have 
a tendency to that form, and, in a few instances, affect the pen- 
tagonal shape of basalt. We were unable, however, to discover 
among them any appearance of articulation in their columnar 
arrangement. It contains but a small proportion of iron; and 
consequently the exposed surfaces of the rock are very slightly 
altered by the oxydation of this metal, which in other places is 
more sensibly observed. 
Of the many interesting minerals to be found at this place, 
stilbite associated with calcareous spar is the most abundant. 
This mineral, forming numerous yeins in the amygdaloid near 
the base of the precipice, presents, in the open interstices 
of the rock, beautiful projecting masses composed of long 
fasciculated crystals of a flesh-red, and sometimes straw-yellow 
color, When crystallized, it is in elongated, rectangular, four- 
sided prisms, terminated by tetrahedral pyramids, 
The calcareous spar is curiously scattered over the surfaces 
of stilbite in acute rhomboids, which are often hemitropic, deeply 
striated upon the faces of cleavage, parallel to their horizontal 
diagonals, and of uncommon magnitude and beauty. These crys- 
tals, usually colorless and transparent, are in a few instances of a 
rich honey-yellow appearance, In breaking the various masses 
which are scattered along the shore, it is not unusual to meet with 
one, which, composed of yellow stilbite externally, contains within 
numerous crystals of calcareous spar lining its walls in rhomboids, 
which, haying their faces deeply indented by the projecting pyr- 
amids of the stilbite on which they are implanted, were obviously 
deposited subsequently to the formation of that mineral, 
