174 MELLO: MICROSOOPIOAL STRUCTURE OF ROCKS. 
rocks also have been so greatly altered, that although they may have 
been once highly fossiliferous, all trace of organic existence has been 
removed and they present a mass of mere crystalline material. 
Calc tuif, which is really a chemical precipitate, will often be 
found upon microscopical examination, to contain remains of con- 
fervse and diatomaceae which have been enclosed in it, upon dissolv- 
ing it in hydrochloric acid the confervas may be found in the residue, 
whilst the diatoms will have been dissolved. Oolitic limestones show 
an interesting structure under the microscope, the concentric character 
of the grains and the accumulation of the crystalline prisms of calcite 
around a nucleus, a grain of sand or a foraminifer, is well seen. 
Some oolitic grains will be found to be concentric, others radiated, 
and a third class recrystallized. 
Chalk will abundantly repay the microscopist who will devote 
time to its study. A vast abundance of foraminifera, fragments of 
shells, polyzoa, echinodermata, and sponges will be found to consti- 
tute this rock, which is a perfect treasury of organic wealth. 
Here this outline of an important branch of microscopical research 
must come to a close, much more might have beensaidupon every branch 
of the subject, some points have been altogether omitted which 
might have been alluded to, but enough, 1 trust, has been said to 
show what a wide field here lies open to the diligent student, on 
every side questions are yet awaiting answers, and beyond all doubt 
many cannot receive them until the microscope has been carefully 
employed. For details and for a full investigation of the subject 
the student must be referred to the elaborate works and papers of 
such men as Sorby, Bonney, Teale, Rutley and others, in this country, 
and to those of Zirkel, Rosenbusch, Renard, Fonque, Levy, and 
many others on the continent. 
