1888.] 
Dr Stecher on Contact-Fhenomena. 
169 
experience goes. Bearing in mind the statements of Mligge^* that 
“ ahnlich wie stark erhitzte nnd der Abkiihlung ausgesetzte und 
deshalb doppelt brechende Objectglaschen sofort ganz oder nahezii 
isotrop werden, wenn das Glas zerspringt,” the anomalous double 
refracting glass of natural pitchstone returns to its normal isotropic 
state as soon as the glass develops perlitic fissures, I shall try to 
interpret the structure of the quartz grains we have been describing 
as the effect of a strain with the tendency to split each quartz crystal 
into diverse grains, differing in their optical orientation. This ex- 
planation is fully borne out by the following arguments. In the outer 
portions of the quartz, where it might seem to have been in contact 
with the magma, and therefore to have been exposed to a specially 
powerful strain, the optic anomaly exi3lained above is sometimes 
repeated on a minute scale. The supposition that the quartz crystals 
had been exposed to a strain will be afterwards discussed. Another 
fact observed, as proving the pyrogenic origin of the quartz, and 
otherwise illustrating the effect of a straining force, should be men- 
tioned. The very few crystalline quartz grains that do not exhibit 
between crossed nicols anomalous phenomena, represent, as a rule, 
the smallest individuals ; these, besides, are more or less free from 
enclosures, while the crystals that give anomalous phenomena are 
rich in gas- vesicles and “stone-cavities” with fixed vesicles. These 
“stone-cavities” often exhibit the dihexagonal form of the including 
quartz, the crystallographic axes being parallel to those of the latter ; 
and thus undoubtedly prove their glassy nature. All the inclosures 
are sometimes arranged in rhombohedral planes, parallel to which 
there occur in one specimen microscopical fissures that might suggest 
either cleavage-planes or gliding-planes. Eeturning to the conclusions 
we arrived at respecting the quartz, and relying upon the state- 
ments of Mligge,f who has pointed out a distinct relation between 
the planes of structure and those of twinning, there are already 
strong reasons to suppose that in the rocks we are considering the 
twinning of the augites and felspars is nothing but the effect of strain. 
As further results from observations, such a tractive power must 
* Neues Jahrbuch fur Mineralogie, B.B. iv. p. 590. See also Quart. Jour. 
Geol. Soc., xl. p. 343, where Rutley describes analogous phenomena in some 
obsidians. 
t A^eues Jalirhuch fur Mineralogie, 1883, i. p. 54. 
