Me LI e PEP eT a TERS Ce OT Pee eee e Pon ED eS PRE Ag cs ee T 
Te ee AES MP ay 
1878. } The Microscope in the Examination of Rocks. 17 
microscope appears as a mass of debris of rocks altered by me- 
chanical means and pseudomorphical actions. 
The study of thin sections of rocks has also widened our 
knowledge of the more frequent occurrence of certain minerals 
as micro-mineralogical accessories, as magnetite, menaccanite, 
apatite, hornblende, tourmaline, nepheline, nosean, microlites, 
and many others. 
It is also due to microscopical researches that crystallography 
and mineralogy have been abundantly enriched in facts which may 
be of the greatest importance for their development as sciences. 
What we formerly thought to be a single crystal has shown itself 
as a number of crystals in position of twin formation. A great 
number of crystals, f y quartz, have been found to be porous, 
the pores filled with liquid, most likely water and carbonic acid, 
and these pores are the most frequent if quartz occurs in granite 
or syenite. 
Orthoclase presents under the polariscope two systems of bands 
crossing each other at right angles. Labradorite is filled with 
menaccanite and magnetite; and mica and magnetite generally 
pierced with apatites when occurring in granites, or in diorites. 
It is not the intention of the writer to describe all those results 
of micro-lithological researches which within a few years have re- 
organized lithology and richly contributed to geology, mineralogy 
and crystallography. The remarkable work of Prof. Zirkel, forming 
the sixth volume of the Report of the United States Geological 
Exploration of the Fortieth Parallel, under the direction of Prof. 
Clarence King, Geologist-in-charge, will demonstrate at once the 
importance of thin section in lithological researches. 
Paleontology likewise has derived a great many new facts, as 
will be seen in a forthcoming volume of the “ Palzontology of the 
State of New York,” by Prof. James Hall. A great number of 
sections of corals and sponges and other fossils have been prepared 
and illustrated. The result derived from its perusal will show 
that palzontology also has progressed as much as lithology by 
the adoption of thin sections and the microscope as a means for | 
the study of fossils. 
O VOL. XIL.—=nNo. 1, 
