1895.] . Geology and Paleontology. 661 
of the quartz) underwent partial hydration, but, held in the vise-like 
grip of the surrounding rock, were unable to expand to the full ex- 
tent of loss of cohesion. When freed from compression, expansion and 
further hydration took place, the mass became spongy, and, freely ab- 
sorbing water, fell into sand and gravel. - This idea led to a series of 
experiments, and from an average of several determinations, Mr. Mer- 
rill obtained an approximation of 1.88, which represents the degree of 
expansion which the rock undergoes in passing from its fresh condi- 
tion into that of undisturbed soil a foot beneath the surface. (Bull. 
Geol. Soc. Am., Vol. 6, 1895). 
Dolomites of the Northwestern States.—The Magnesian 
series distributed through southern Wisconsin and Minnesota, extend- 
ing into northwestern Iowa have been studied by C. W. Hall and F. 
W.Sardeson. From paleotological evidence the authors divide the se- 
ries into four alternating formations of dolomites and sandstones be- 
longing to the Upper Cambrian and a fifth of dolomite which may be 
considered a part of the Ordovician. 
As to the origin of the dolomites, the authors do not commit them- 
selves to any theory, but point out that the porous condition of the 
dolomite and the freedom of the sandstones and arenaceous shales of 
the series from the several impurities so universal in recent rocks of 
this character suggest that the original rock mass, which was a lime- 
stone of the same constitution as those now forming within ocean areas 
—that is, a carbonate of lime with a percentage of magnesium carbon- 
ate—has become dolomitic through the removal of the calcium carbon- 
ate. (Bull. Geol. Soc. Am., Vol. 6, 1895.) 
The Silver Mines of Lake Valley, New Mexico.—These 
mines are situated about six miles from the old Sante Fé trail, and 
fifteen miles from the Rio Grande. The ore deposits lie close to the 
surface and are marked by large outcrops of black flint and iron. An 
interesting account of the working of these mines was read before the 
Amer. Inst. of Mining Engineers by Mr, Ellis Clark, in which he gives 
the following theory of the ore-formation: 
“Tt has been held, almost from the time of their discovery, by those 
familiar with the deposits of silver-ore at Lake Valley, that the one 
must have come up in solution from below, that it came along the 
‘blanket’ of iron-flint, and that it was in some way dammed up or 
stopped by the overflow of porphyrite, which may be said, in a general 
way, to. overlie the outcrop of the ‘blanket.’ On the strength of this 
