THE STRUCTURE AND ORIGIN OF LIMESTONES. 
135 
A knowledge of the true mineral constitution is of the 
highest importance in studying the changes which occur in 
fossils and in the investigation of limestones, since the final 
results depend almost exclusively on the original mineral con- 
stitution, and vary very greatly, according as the organism is 
calcite or aragonite. This is due to the fact that calcite is 
in a state of stable equilibrium, and cannot be changed to 
aragonite ; whereas aragonite is relatively in a state of unstable 
equilibrium, can be changed to calcite, and usually is so 
changed, in limestone rocks. This circumstance has given 
rise to a complete difference in the state of preservation of 
many fossils. When they were originally calcite they may 
have been further consolidated, although retaining their original 
structure and optical properties ; whereas when they were 
aragonite they have sometimes been completely removed by 
solution, and in other cases are changed into a mass of crystals 
of calcite, and have lost their original microscopical and optical 
characters. The general structure of various recent and fossil 
organisms was then considered, with special reference to the 
identification of the minute fragments occurring in limestone 
rocks. When they were originally composed of calcite, and 
have retained their original characters, there is usually little 
difficulty in identifying them more or less definitely, according 
to their particular nature, so as to be able to form a good 
opinion as to the constitution of each particular limestone. 
The various facts connected with the disintegration of shells, 
corals, and other organisms are of great importance in study- 
ing limestones, since without an adequate knowledge of the 
manner in which they decay and fall to pieces very inaccurate 
conclusions might be formed respecting the origin of calcareous 
deposits. The results mainly depend on original structure, 
and on whether they are composed of calcite or aragonite. 
The next questions considered were the manner in which the 
external form of minute fragments is preserved in limestone, 
and the various chemical changes occurring after deposition 
or consolidation. When their structure is preserved there is 
no difficulty in recognising the fragments; but when they 
have become almost equally crystalline with the surrounding 
material, their outline is shown by a thin layer of dirt, or by a 
zone of finer crystalline texture. After deposition and con- 
solidation certain changes have often occurred, the chief of 
which are the 'replacement of more or less of the lime by 
magnesia or by iron compounds. After having thus estab- 
lished the general principles necessary for their accurate study, 
Mr. Sorby entered on a description of our various English 
limestones, in descending order. 
The main object was to ascertain, as far as possible, the exact 
