42 T. 8. Hunt on the Criticisms of Prof. Dana. 
tallized mineral species frequently includes a large and even a 
predominating portion of another, and the combination is then 
considered by many as an example of partial pseudomorphous 
alteration. In such instances, remarks Delesse, the question 
arises whether we have to do with the results of envelopment, 
or of chemical alteration ; to resolve which it becomes necessary 
to study carefully the pro oblem of envelopment. He then pro- 
ceeds to show that the enveloped substance is, in some cases, 
erystalline (and arranged either symmetrically or asymmetri- 
cally with regard to the enveloping mass), and in other cases 
amorphous, and enclosed like the sand-grains which predominate 
in the calcite crystals of Fontainebleau: The difficulty im 
deciding whether we have to do with envelopment or with 
epigenesis increases when the enveloped mineral becomes so 
abundant as to obscure the enveloping species, or when it 
becomes mixed with it in so intimate a manner as to seem one 
with the latter, (se fondre insensiblement avec lui). The propor- 
tions of the enveloped and the enveloping mineral, we are 
told, may so far renee that the one or the other is no longer 
recognizable. - ‘As the forces which downing erystallization 
have a great asides the ae mineral is = 
pages iy 339, 341, 53]. 
Our author then proceeds to tell us that having carefully- 
idions in numerous specimens, the supposed mica-pseudo- 
morphs. of iolite, andalusite, sig pyroxene, hornblende, ete., 
he regards them, a as in all cases, examples of envelopment, and 
expresses the opinion that we must omit from our lists a great 
number of the so-called pseudomorphous minerals, especially 
among the silicates. The final result of the process of envelop- 
ment is, according to Delesse, this—to give rise to mixed min- 
eral cs owing their external _ = at crystallizing 
