1887] Mineralogy and Petrography. 177 
for every intrusive one, if difference of structure is due merely 
corresponding intrusive equivalents. This is accounted for by 
Rosenbusch on the assumption that during the gradual rise of 
the magma in the cracks through which it reached the surface 
enough time was consumed to allow of its separation into strata, 
the heavier, more basic portions accumulating towards the 
bottom, and the lighter portion floating on the top 
The characteristic structure of the effusive rocks is the por- 
phyritic. The porphyritic crystals are supposed to have been 
developed at the period during which the rock was ascending to 
the surface. After it reached the surface another more rapid 
crystallization set in, and the result is a glassy or finely-granular 
ground-mass. 
The effusive ra are divided into two classes, according to 
age. This iš only remnant left of the old classification into 
_ pre-Tertiary cat recent rocks. Here certain characteristics are 
noted in those rocks erupted at an early geological period, which 
distinguish them from the rocks of later eruptions,—e.g., the 
ground-mass, of the older rocks, is more lithoidal in character than 
that of the younger ones, the appearance of their porphyritic feld- 
spathic constituents is different t, etc. Whether these differences 
are of primary or secondary origin is still a matter of doubt. 
The pretertiary effusive a are known as palæo-volcanic, the 
younger ones as neo-volcan 
The palæo-volcanic rocks include the quartz-porphyries, quartz- 
free- opii, eee augite-porphyrites and melaphyres, 
and the pikrite-porphyr 
e full discussion of the different varieties of the palzo- 
cles rocks and the entire discussion of the younger class are 
left to the second part of the book, which is promised to appear 
in a few months. It is of course impossible to give any adequate 
conception of the amount of new material incorporated in this 
volume, or even to mention all the important results reached in 
its consideration. Perhaps the most important of all the advances 
_ have been in the direction of what is now known as dynamical 
metamorphism, by the action of which a massive rock is made to 
assume a schistose structure. This mode of alteration is treated 
in some detail. 
It is needless to say that the very latest ee Soa ste have all 
been critically examined and their teachings made use of in de- 
veloping the new system of classification. It isa gitter wobe 
of note that quite a large proportion `of the most instructive 
papers bearing on this subject have been # shaggngs by Americans. 
Instructors in petrography, and all those to whom a ready 
Sth of German is denied, will be glad to learn that a 
. n of both the first and second volumes of the “ Mikro- 
