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use of the microscope, can be traced back to their original un- 
altered state, from which they have been changed by the action 
of magnesian solutions. 
The metamorphism of rocks produced by gasolytic action, as, 
for example, carbonate into sulphate of lime, &c., has, as yet, 
not been made the subject of microscopical enquiry. 
The foliated schists, quartzites, &c. form by themselves a 
distinct and well-defined class of metamorphic rocks, charac- 
terised by structural peculiarities differing from all previously 
treated of. 
This appears to be due to their crystalline development having 
originated in a solid body, and not from liquefaction ; the 
minerals composing them differ greatly in structure from the 
same minerals when found in eruptive rocks. Instead of, as in 
the latter case, presenting themselves in more or less defined 
crystals, occurring in all positions and at all angles to one 
another, in the foliated rocks they are developed only in one 
general direction, not characterised by well-defined bounding 
planes, but forming a string of drawn-out and irregularly 
bounded crystalline aggregations, presenting a general parallelism 
to one another, as is illustrated by PI. XVII. fig. 8, which shows 
a section of hornblende schist from Connemara. 
The microscopic examination of these rocks proves their 
original sedimentary origin, often showing the contours of the 
original sand grains, and, as Sorby has pointed out, the existence 
of ripple-drift and wave-structure, peculiar to sedimentary rocks 
alone. These rocks appear to have been micaceous and argilla- 
ceous sandstones, the constituents of which have been recrystal- 
lised in situ, owing to molecular action developed in the solid 
rock. 
The quartz of these schists frequently contains numerous fluid 
cavities, indicating that they have been exposed to a pressure 
under which the water, always present in more or less quantity 
in sedimentary rocks, has been entangled and retained during 
the recrystallisation of the quartz. 
The direction of the lines of foliation or crystalline develop- 
ment is that of the lines of least resistance in the rock, which 
commonly will be the lines of stratification, but in cleaved rocks 
will doubtless be those of cleavage. Sorby has alluded to this 
fact by the names of u stratification foliation ” and u cleavage 
foliation.” 
In conclusion, the author of this short sketch hopes that it 
may be the means of attracting attention to the subject, and 
thereby of causing a hitherto almost unexplored field of micro- 
scopic enquiry to be more cultivated ; and leaves it to his readers 
to form a correct estimate of the justness of the sneering assertion 
that “ mountains should not be looked at through microscopes.” 
