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272. General Notes. [March 
the surface of the earth, it is a matter of’no little surprise that 
sections of these bombs have not been more thoroughly inves- 
tigated by means of the microscope and the other appliances 
now so generally made use of in the attempt to discover the 
origin of rocks and minerals. The most satisfactory article which 
has thus far appeared on this subject is that of Bruno Mierisch,* 
working under the supervision of Professor Zirkel at Leipzig. 
Eighty specimens of these bodies belonging to the collection of 
the University of Leipzig were examined. As might be expected, 
the results reached are exceedingly interesting. According to 
Mierisch the bombs may be divided into two great classes: (1) 
those consisting of broken pieces of older lavas, which are in- 
cluded in the younger lavas, and (2) the limestone or silicate 
bombs, in the druses of which the crystallized minerals, as men- 
tioned above, are found. It is to the latter class that the present 
writer confines his attention. This class can be subdivided into 
limestone bombs and silicate bombs, and the latter of these again 
into (1) those in which the minerals are zonally arranged, and (2) 
those in which this arrangement is wanting. Under the micro- 
scope the limestone bombs are seen to consist of grains of cal- 
cite and an olivine mineral, which analysis proves to be forsterite, 
—the pure magnesium oliyine. A noteworthy fact in this con- 
nection is the entire absence of even a trace of calcium in the 
forsterite, and the existence of the merest trace of magnesium 
in the closely-associated calcite. When druses occur in these 
occurring in them, and describes in detail its appearance, micro- 
scopical characteristics and associations. Here again we find 
most important of which can be noticed. In the calcite of the 
limestone bombs glass inclusions were detected. These, accord- 
ing to the author, cannot be considered as secondary in origin, 
because not a trace of glass was detected in the ground-mass 
of any section examined. Consequently the calcite must have 
*Tschermak’s Min, u. Petrogr., Mitth. viii, 1886, P> 114. 
