499 ADDRESS OF T. STERRY HUNT. 
to its own crystalline form a large portion of some foreign material, 
which may even so mask the crystallizing element that this be- 
comes overlooked, as of secondary importance. The substance 
which, under the name of houghite, has been described as an al- 
tered spinel, is found by analysis to be an admixture of véllknerite 
with a variable proportion of spinel, which, in some specimens, does 
not exceed eight per cent., but to which, nevertheless, these crystal- 
loids appear to owe their more or less complete octohedral form. * 
The above characteristic examples of symmetrical and asymmet- 
rical envelopment are cited from a great number of others which 
domorphists regarded as results of partial alteration. Thus, in 
the case of associated crystals of andalusite and cyanite, Bischof 
does not hesitate to maintain the derivation, of andalusite from 
the latter species by an elimination of quartz; more than this, as 
the andalusite in question occurs in a granite-like rock, he sug- 
gests that itself is a product of the alteration of orthoclase. In 
like manner the mica, which in some cases coats tourmaline, and 
in others, fills hollow prisms of this mineral, is supposed to result 
from a subsequent alteration of crystallized tourmaline. 50 in 
the case of shells of leucite filled with feldspar, or of garnet en- 
closing epidote, or chlorite, or quartz, a similar transformation of 
the interior is supposed to have been mysteriously effected, while 
the external portion of the crystal remains intact. Again the ag 
gregates of tinstone, quartz and orthoclase having the form of the 
latter, are, by Bischof and his school, looked upon as results of a 
partial alteration of previously formed orthoclase crystals. It 
needed only to extend this view to the crystals of calcite enclos- 
ing sand-grains, and regard these as the result of a partial alter- 
ation of the carbonate of lime. There is absolutely no proof 
that these hard crystalline substances can undergo the changes 
supposed, or can be absorbed and modified like the tissues of @ 
living organism. It may, moreover, be confidently affirmed that 
the obvious facts of envelopment are adequate to explain all the 
cases of association upon which this hypothesis of pseudomorphism 
by alteration, has been based. Why the change should extend to 
some parts of a crystal and not to others, why in some cases the 
exterior of, the crystal is altered, while in others the centre alone 
ot ae, eae 
* Report Geol. Survey of Canada, 1866, pp. 189, 213. Amer. Jour. Sci., II, i, 188. 
