374 Haworth on the Archtean Geology of Missouri. 
In A^^ril of the same year Signor B. Lotti ' described certain 
Italian rocks which teach the same lesson. In 1887 Karl Dal- 
mer- described the quartz trachytes and other rocks from Cam- 
piglia in which he finds additional evidence that cr^'stallization 
dejoends principally upon the conditions of heat and pressure at 
the time of solidification, rather than upon the age of the rock 
magma. 
((/) Classijicatlon of the porphyries. 
In the classification of the porphyries it is customary to base 
the first general divisions on the mineral constituents of the rocks, 
considered, and, a portion at least, of the subdivisions on the 
structure of the ground mass. In this way Rosenbusch divides 
the paliEO-volcanic rocks into five general divisions, or families; 
the quartz-porphyries, the quartz-free-porphyries, fhe porphy- 
ry tes, the augite-porphy ry tes, and the pickrite-porphyry tes. The 
first three of these families are well represented in Missouriy 
and the fourth very probably existed at one time, although at 
present, as before stated, the epidote is the only representative 
of the original bisilicate, whatever it may have been. 
The quartz porphyries include representatives of three of the 
sub-divisions given by Rosenbusch. These three are: micro- 
granites, granophyres, and felsophyres, the rocks which orig- 
inally were vitrophyres having been completely devitrified.. 
The quartz-free porphyries can well be placed in a single sub- 
division, the orthophyres; the minor sub-divisions, such as augite 
orthophyre, biotite orthophyre, etc, have not been discovered,, 
and probably do not exist. 
The porphyrytes cover a very large area. The different 
varieties grade into each other so that division lines cannot easily 
be drawn between them. Some of them have quartz as an es- 
sential constitutent, and others do not. Some have an abundance 
of epidote, and others have none. Leaving out of consideration,, 
the augite porphyrytes — and we have to do this, because we 
can say nothing about them with certainty — we may represent 
the classification of the whole of the palceo-volcanic surface rocks 
1 R. Com. Geol. D'ltalio Boll., April, 1886, p. 73. Reviewed by ProL 
J. D. Dana in Am. J. Sci., vol. 32, p. 239. 
2 Neues Jahrb, 18S7. 11 Sand, pp. 206-221. 
