364 
The American Geologist „ 
June, 1896 
ites containing a little olivine and presenting an ophitic 
structure easily understood in the case of the thin sheets. 
Thus it seems to me there is little doubt that the ophites 
(labradorites) of this region represent lava sheets, the points 
of issue for which were at Segalas. 
I have recently examined many of the ophitic regions of 
the Pyrenees other than those of Segalas, and have shown 
what intense metamorphic phenomena the secondary lime¬ 
stones present at their contacts. I have concluded from this 
that, as with the lherzolites which they accompany, the 
ophites are of intrusive origin. This view receives confirma¬ 
tion in the facts observed at Segalas, which show that the 
same magma, spread over the surface of the secondary lime¬ 
stones or embracing them in its mass, is incapable of develop¬ 
ing in them any of the numerous minerals which form the 
accompaniments of the ophites properly so-called. These con¬ 
clusions agree furthermore with my recent memoir on the 
contact phenomena of volcanic rocks. 
There is room, therefore, to consider two categories of ophitic 
rocks in the Pyrenees, one of an effusive origin, presenting no 
metamorphic phenomena at their contacts, the other intrusive 
and developing intense mineralogical changes in the ro_*ks 
amongst which they are found. There can be no doubt that 
these two categories of ophites come from a single magma. 
It remains to find by minute study the differences, however 
slight they appear at first, which may exist, in a mineralogi¬ 
cal point of view, between these two categories of rocks. This 
question will form the subject of later work. 
Paris, 20 Jan., 1896. [Comptes Rendus des Stances de VAcademie des 
Sciences.] 
FROZEN STREAMS OF THE IOWA DRIFT 
BORDER. 
By Andrew Gordon Wilson, Hopkinton, Iowa. 
The valleys of the streams of Delaware county, and of 
other counties in Iowa that lie to the north and southeast of 
it, were considerably deeper in preglacial days than at pres¬ 
ent. According to Prof. Chamberlin,* the old Mississippi 
*Professors Chamberlin and Salisbury, U. S. Geol. Survey, Sixth 
Annual Report, p. 223. Cf. also data by Frank Leverett, Journal of 
Geology, Oct.-Nov., 1895. 
