1887] Mineralogy and Petrography. ° 173 
‘observed facts, and the number of minds devoting themselves to 
the application of these facts to the study of the rocks them- 
selves and to their origin became greater, new fields were ex- 
plored and new facts were observed, which seemed to contradict 
many of the deductions obtained by the earlier investigators, 
and rendered a revision of their views necessary. This is es- 
pecially true in regard to the classification of the massive rocks, 
the separation of these rock-masses into families,—into groups, 
all the members of which are characterized by certain general 
properties which mark them as belonging to the same group, 
and distinguish them from the members of other groups. Until 
we know the history of every rock on our globe and can trace it 
back to its origin through all the changes which it has under- 
gone since its first existence as a distinct portion of the original 
molten magma, all attempts at a perfect classification of these 
bodies must be futile, and the classification itself must be an ar- 
tificial one. As we can never expect to know everything relating 
to even a single rock type, so we can never expect to possess a 
perfect classification of rocks. The differences in structure pro- 
duced during the solidification of molten masses by slight dif- 
ferences in the conditions of temperature and pressure under 
which they cool are so great, and our knowledge of the effects of 
these differences is so limited, that we must be content, with that 
classification which best conforms with the facts known and incor- 
porates them in a consistent whole. In no maiural science can we 
hope for more than this, and least of all can we hope for it in pe- 
trography, the materials of which are the embodiments of the ac- 
tion of largely unknown conditions on substances of whose original 
nature little can be positively known. It is safe to affirm that no 
classification of rocks ever proposed has met with such general 
acceptation as that of Professor Rosenbusch’s as developed in 
the first edition of his “ Massige Gesteine.” That this, however, . 
did not fully meet the requirements of the rapidly-growing science 
has been acknowledged for some years past. at classification 
was based upon the fundamental notion that rocks erupted before 
the Tertiary period in geological time, in consequence of certain 
conditions then prevailing, possessed structural and mineralogi- 
cal characteristics which distinguished them from those of more 
recent age. This, then, was taken as the ground upon which to 
separate all massive rocks into two great divisions,—the pre-Ter- 
tiary and the recent rocks. These were again subdivided ac- 
cording to their structure into granular, porphyritic, and glassy 
rocks; and finally the members of the subdivisions were classi- 
fied by groups in accordance with their mineralogical composi- 
tion. Recent investigations, especially those of Italian and Amer- 
ican workers, have shown that structure is dependent not upon 
the age at which rock-masses were formed, but ra’ the 
geological conditions under which they were produced. A mass 
