MAGMATIC ALTERATION OF CERTAIN MINERALS 259 
rocks; that in the most glassy forms of the volcanic rocks the 
alteration is uncommon; and lastly that melted hornblende 
recrystallizes as augite. 
These arguments are of great weight and will be discussed 
at greater length later on, Leaving them aside for the present, 
I would call the reader’s attention to a fact often observed which 
they not only fail to explain but with which they seem to be 
quite at variance. This is the preservation of the original form of 
the hornblende or biotite crystal, with its sharp angles and straight 
edges, in a magma which has been in motion around it, as is 
shown by the flow structure and fragmentary hornblendes with 
their fracture surfaces unaltered. That the molten or dissolved 
zone surrounding the crystal demanded by these theories could 
possess sufficient cohesion or solidity to preserve its exact origi- 
nal form, unchanged by the, in many cases, powerful disrupting 
action of the moving magma, is to me quite inconceivable and 
passes the bounds of reasonable conjecture. Such a zone must 
be, in the nature of the case, more or less fluid, often as fluid as 
the surrounding magma, and hence easily subject to all kinds 
of distortion and disruption by the current. Yet such distor- 
tions as are expected are not found. In many cases the exact 
original form is preserved, and in the cases where the original 
form is lost the outlines are merely rounded by the removal 
of the outer portion, and the greater part, or the whole, of 
the remaining mass is composed of the granular alteration 
products. 
The supposed fusion of the crystals has been accounted for* 
by the rise in temperature of the rock-mass on solidification 
which is demanded by theory and which has been actually 
observed.? But it has not been shown that the rise in tempera- 
ture is sufficient for the fusion, nor why such an effect does not 
take place on the augites, which would destroy any zonal or hour- 
glass structure. Furthermore this hypothesis cannot explain the 
very numerous cases of alteration which took place while the 
*Lacorio, Natur d. Glasbasis, Min. Pet. Mitth. VIII, 463, 1887. 
2 ROTH. Der Vesuv., Berlin, 1857, 304. 
