314 Minerals from New. Holland. 
similar to some of the specimens met with in the 
more ancient lavas of Vesuvius. "They do not puo 
groups of united, divergent crystals. 
Rhombohedral Quartz. — I was so fortunate as to 
find among this collection; besides several of the sub- 
species of this mineral, two or three specimeris exhib- 
iting the primary obtuse rhomboid in great perfec- 
tion, and of considerable size; some of them measur- 
ing half an inch across their planes. They present 
highly polished surfaces, are transparent and color- 
less; and it is evident, I think, that they cannot be 
regarded as pseudomorphs of any other mineral, from - 
the agreement in the value of their angles, as meas- 
ured by the goniometer, with those of the obtuse 
rhomboid established as the fundamental. form of 
the species, as well as from the fact, that the crys- 
tals are imbedded in cacholong, which now occupies 
the interior of the geodes. If pseudomorphous, the 
mineral of which they are imitative, must have been 
removed, so as to admit of a subsequent infiltration 
of the cacholong. It is more probable, that the 
quartz and cacholong were formed at the same time, 
thé latter affording a soft matrix in which the crys- 
talline molecules could freely arrange themselves, 
according to the laws which governed them. 
Two other modifications of rhombohedral quartz 
were met with among these specimens, of compara- 
tively rare occurrence. In one, the primary rhom- 
boid, by a very deep truncation of its lateral solid 
angles, has given rise to long, six-sided prisms, ter- 
minated by trihedral summits, there being no. trian- 
gular replacements upon the upper edges of the 
rhomboid, by which the usual six-sided pyramids are 
