518 Forbes — The Microscope in Geology. 



the g-raius of sand lias been merely deposited from a solution of 

 carbonate of lime whiclL has percolated through it, and in otherwise 

 unaltered limestones it is common to find microscopic veins of calc- 

 spar, due to minute cracks or fissures, filled up in a similar manner. 

 Frequently however, such infiltration is accompanied by an entire 

 change in the chemical composition of the rock itself ; thus the beds 

 of Cleveland ironstone have been proved by Sorby's microscopical 

 researches to have been originally shell-limestones converted into 

 carbonate of iron by the action of ferruginous solutions, the frag- 

 ments of the original shells being still distinguishable in aU stages 

 of conversion ; in the same manner he has proved the Magnesian 

 limestones of the Carboniferous and Devonian ages, as well as the 

 Permian dolomites, to have been originally common limestones, or 

 aggregations of organic debris, the particles of which, by the use of the 

 microscope, can be traced back to their original unaltered 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, etc., has, as yet, not been 

 made the subject of microscopical enquiry. 



The foliated schists, quartzites, etc., form by themselves a distinct 

 and well-defined class of metamorphic rocks, characterised by struc- 

 tural 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 seen in 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 argillaceous sandstones, 

 the constituents of which have been re-crystallised 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 sedi- 

 mentaiy rocks, has been entangled and retained during the re-crys- 

 tallisation of the quartz. 



The direction of the lines of foliation or crystalline development 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 "stratification foKation" and "cleavage foliation." 



