THE STRUCTURE OF ROCK MASSES. 
239 
nally invisible or but faintly distinguishable, when the glass 
had been for some time exposed to a low heat in the flue of 
the annealing oven. Had this specimen been kept long enough 
in the hot flue, it would eventually have become altogether 
devitrified or converted into a crystalline stony mass (Reau- 
mur’s porcelain), in which, however, the parallel structure due 
to the strise of fusion can still be distinguished either by the 
naked eye or under the microscope, owing to the general 
direction and parallelism of the longer axes of the crystals 
which compose the mass. 
' What is thus artificially produced on the small scale can 
be seen on the large scale in nature in obsidian and other 
glassy lavas ; and in most volcanic countries hand specimens 
can be obtained showing all the stages, exactly as in the case of 
glass. The crystals of the mineral components of the lavas 
appear first along the lines of striation, and go on forming 
until the whole mass has become devitrified and is entirely 
crystalline, and resembles PL LXXIII., fig. 9, which represents 
a hand specimen from the apparently bedded and intercalated 
lavas (which form so peculiar a feature of this part of South 
America) near the river Mauri, where it forms the boundary- 
line between the republics of Peru and Bolivia. In this speci- 
men the rock has lost all trace of its originally glassy or 
vitreous appearance, and having become completely devitrified, 
is seen to consist mainly of felspar crystals along with a little 
augite, quartz and an occasional plate of mica (a trachydolorite); 
yet, as is seen in the figure, a distinct parallel structure (due 
to the strise of fusion) is still preserved in the rock, which, 
being the lines of least resistance in it, renders it much more 
fissile in this direction than across the grain, exactly as is 
commonly found to be the case in a normal or metamorphosed 
rock of true sedimentary origin : in fact, some of these rocks, 
which underlie conformably oolitic and other strata of still later 
age, and which show themselves for miles intercalated con- 
formably in the strata, have been mistaken for true sedimentary 
beds, and in one instance described as sandstone (which never 
could have occurred had their mineral nature been examined), 
although they are easily traced to the active volcanoes in the 
vicinity from which they have been emitted. Even when such 
rocks are found to be extremely fine-grained in texture, the 
parallel structure due to pre-existing striation, although often 
very difficult to distinguish on the rough surface of fresh 
fracture, is commonly found to show itself very distinctly on 
the surface of the rock when weathered. 
Any geologist who has had the opportunity of studying such 
rocks in the field, cannot but consider it probable that the so- 
called granitic gneiss or gneiss granites owe their structure to 
milar causes. 
