Forbes — The Microscope in Geology. 517 



Under this class are included most beds of gypsum, rock-salt, and 

 other saline bodies, as well as travertine, silicious sinter, flint, infu- 

 sorial slates and earths, limestones, etc., many of which have been as 

 yet but very superficially examined. 



In the microscopic investigation of such rocks as owe their origin 

 to the developement of organic life, very considerable progress has 

 been made, with correspondingly important and interesting results. 



As early as 1836 Ehrenberg proved that large rock masses were 

 built up of the carapaces of minute silicious infusorige, and more 

 lately Sorby has done good service by his investigation of lime- 

 stones : these he has proved not to have originally possessed any 

 crystalline structure whatsoever, but to have been deposited as mere 

 mechanical aggregates (aptly termed by him, organic sands or clays) 

 formed of the debris of calcareous organisms, which admit frequently, 

 not only of being recognised, but of having their relative proportions 

 determined. The comparison of the microscopic structure of the 

 organisms in Chalk, with those now forming in the depths of the 

 Northern Atlantic Ocean, indicates that there is an immense deposit 

 now in the course of formation, quite analogous to what had pre- 

 viously taken place in the seas of the Cretaceous period ; and the 

 same able observer has shown that the reason why certain calcareous 

 organisms are found so well preserved, whilst others had disappeared 

 or become entirely disintegrated, was from the carbonate of lime in 

 the first being in the form of the stable calcite, whilst in the latter it 

 was present as instable Aragonite. 



When a calcareous rock has undergone cleavage, the microscope 

 shows a distortion of its particles and organisms, just as in a cleaved 

 slate, though in a much less degree ; the measurement of such distor- 

 tion serves as a basis for estimating the amount of compression 

 undergone. 



With the exception of having briefly referred to the alterations in 

 igneous rocks, subsequent to their solidification, and the cleavage of 

 sedimentary beds, all the classes of rocks treated of have been con- 

 sidered in their normal or unaltered condition. It remains now to 

 direct attention to the use of the microscope in the study of subse- 

 quent alteration or metamorphism of rocks. 



Many sedimentary beds become more or less indurated, at points 

 where they are cut through by eruptive dykes ; thus the Coal-shales 

 and clays of Staffordshire are found altered into a hard rock with 

 conchoidal fracture, or even into porcellanite, when in immediate 

 contact with basaltic dykes. An examination shows no change in 

 mineral or chemical composition beyond the expulsion of the water 

 always contained in such beds, and sections of such rocks are often 

 seen to be quite identical in structure with those of common stone- 

 ware made from the same clays, the only difference being that the 

 latter is usually more porous from not having been submitted to the 

 pressure which rocks baked in situ would experience. 



The alteration of rocks produced by infiltration may or may not 

 be accompanied by chemical changes. Thus a section of Calcareous 

 grit often shows that the calcite filling up the interstices between 



