MELLO: MICROSCOPICAL STRUCTURE OF ROCKS. 
173 
Turning to the second or calcareous group of aqueous rocks, 
we have before us a large and very interesting field for microsco- 
pical research. The greater part of the limestone rocks is organic 
in its origin, that is to say, it is built up out of comminuted shells, 
corals, and foraminifera. We have but to look at a slab of the 
common fossiliferous hmestone of Derbyshire to perceive without 
any instrumental aid that it is one dense mass of broken organisms, 
encrinital stems, fragments of shells, etc., and the microscope 
will show that hner-grained parts are equally full of minute 
foraminifera, and other remains of an abundant fauna existing 
once in the carboniferous sea. It is the same with many 
other limestones. Dr. Sorby in his very interesting address 
to the Geological Society of London, in 1879, entered with 
some minuteness into the question of the origin of limestones 
and their structure, and he observed that in studying these rocks 
a knowledge of the differences in the mineral constitution of cal- 
careous organic bodies is very essential. In order to gain 
such knowledge both chemistry and the microscope have to be em- 
ployed. He has pointed out that in the structure of shells the two 
forms of carbonate of lime aragonite and calcite occur, sometimes also 
some phosphate. One somewhat important point connected with 
this is that whereas calcite is a stable, aragonite is an unstable condi- 
tion of carbonate of lime ; shells in which calcite is the prevailing 
material would be more readily preserved than those formed of 
aragonite, and the absence therefore of the remains of species in 
which aragonite occurs would not prove that such species were non- 
existent in the fauna of a given formation. "Thus then a knowledge 
of the structure of fossil shells and other organisms necessarily 
assists us much in studying the fragments which constitute so large 
a part of many limestones." 
Again, there are limestones whose origin is distinctly chemical, 
the carbonate of lime, or magnesia, has been precipitated fi'om 
solution, or in the case of dolomite, the carbonate of lime may have 
been partially replaced by magnesia. Microscopical structure alone 
will not suffice, however, to enable us to determine whether the rock 
under examination is of organic or chemical origin. Some limestone 
