3/0 TJic American Geologist. 'no, ib98 
the surface, which, by tlie peneplain theory, call njr special 
causes vvliose o])eration is apparently not otherwise proved, 
and which, in part, appear to be hardly probable. The theory 
of the peneplain calls for a "nearly featureless plain"; the 
alternate hypothesis of beveling calls merely for a greatly re- 
duced, but still markedly irregular surface. To some the 
difference between these two hypotheses may seem slight, but 
really it is great; for, after the rounded features of maturity 
are reached, the advance to such old age topographic features 
as the peneplain demands, calls for immense periods of time 
with land standing at nearly the same level, conditions which 
seem at variance with the facts which geologists have been 
collecting in the last half century. 
STUDIES ON AN INTERESTING HORNBLENDE 
OCCURRING IN A HORNBLENDE GABBRO, 
FROM PAVONE. NEAR IVREA, 
PIEDMONT, ITALY. 
By Frank R. Van Horn, M. S., Ph. D., Case School of Applied Scieuce, 
Cleveland, Ohio. 
This hornblende gabbro* consists of the following min- 
erals with an approximate estimate of their percentages: 
plagioclase, mostly b)'townite, 33, hornblende 27, diallage 
and hypersthene 25, magnetite and spinel 15. After the 
plagioclase, the brown hornblende is the most important 
mineral of the rock and makes up about 27 per cent, of the 
same. It generally occurs in irregularly shaped but compact 
patches, and only occasionally has an approximately idio- 
morphic form in the prismatic zone. This is somewhat pe- 
culiar as it is one of the most basic constituents of the rock, 
but the lack of idiomorphism may perhaps be explained b}- 
the high percentage of alkalies which this mineral contains, 
^girine, arfvedsonite and other minerals with a large per- 
centage of alkalies which occur in elaeolite syenytes show 
*For further description of the rocks of this region, see Tschermak's 
Mineral, und Petrogr. Mitth., Bd. XVII, Heft. 5; Frank R. VanHnrn, 
"Petrographische Untersurhungen fiber die Noritischen Gesteine der 
Umgebung von Ivrea in Oberitalien." 
