210 ‘On the Venis of Hot Vapor in Tuscany. 
ik 
same aqueous matrix. When, however, we recede from the im- 
mediate point of contact, we have not only very different forms 
in the matrices of the altered and the eruptive rocks, but an es- 
sential difference of composition and structure. Pilla has indeed 
cited instances just as notable of the conversion or metamorphosis 
of the strata by gabbro rosso, as by granitic, pyroxenic and por- 
phyritic rocks.* One of those examples is seen in the spot ealled 
Botro del Ribuio near Serazzano, where the spheroidal “ gabbro 
rosso” has thrown the strata of macigno into a vertical position, 
and has changed them into jaspers of blood-red color, highly 
charged with silex and oxyd of iron. 
If, indeed, the argument about transitions from the rock whieh 
has been the agent of alteration into the strata which are altered, 
- beadmitted, we must re-open elementary questions in the physics 
of geology which I had supposed were long ago set at rest. We 
may in that way be led to abandon many conclusions at which 
we had arrived, in refuting the doctrine respecting certain rocks 
of Cornwall, Norway, and other tracts which were believed by 
some authors to prove transitions from granites to slates, and thus 
to indicate a common origin of these two classes of rock! If 
this method of ‘reasoning be again entertained, (as it seems to 
me it is by M. Savi,) then many of the inferences which geolo- 
gists have drawn concerning the posterior intrusion of granite and 
other igneous rocks amid depositary strata will be invalidated. 
For, although there are numerous examples of such phenomera, 
which no skeptic can assail, still there are frequent cases where it 
is impossible to define the precise limit between the erupted molten 
matter and the altered rock. It is indeed in the very nature of 
the phenomenon that such should happen, and the time of practi- 
cal geologists can be better employed than in disputing upon such 
points. Some persons may indeed argue that many varieties of 
traps and amygdaloids were to a great extent evolved from the 
melting of the preéxisting strata in the crust of the globe, and I 
am quite ready to admit that such may have been the case. But 
with 
a 
the other ophiolitic or serpentine rocks, which having acted as partial centres of eleva- 
i i ical, ed, and rugged mounts, detached from 
one another (p. 39). He describes the copper of Monte Catini as lying in a true vel, 
whi iarity of being contemporaneous with the associated gabbre, both 
r to the sedimentary strata (p. 40). ; 
+ Trattate di Geologia, Part 1, p. 510, ; 
{EF LEE Reale hee) ee ORL 
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