1897,] Petrography. 1051 
the Marquette district was carefully described years ago by Wads- 
worth. The only additional fact of importance added by the authors 
with respect to this rock concerns its composition. An analysis of a 
very fresh specimen gave : 
SiO, TiO, Al,O3 Cr203 Fe,03 FeO MnO rd CaO SrO BaO MgO K0 Na,0 CO, P20; H0 Total 
89.37 .66 4.47 .68 4.96 9.13 .12 3.70 tr tr 26.53 .26 .50 1.23 17 7.95 = 99.94 
A few hornblende-schists met with in certain portions of the area 
studied are shown to be altered igneous rocks, possibly recrystallized 
lavas. 
The micaceous schists are well banded rocks with a distinct dip and 
strike. They are often interlaminated with thin bands of hornblende- 
schists with which they are perfectly conformable. These micaceous 
schists embrace muscovite-schists, biotite-schists and feldspathic biotite- 
schists, most of which are so much decomposed that their original com- 
position is difficult to determine. It is probable that they are crystal- 
lized acid lavas or tuffs. 
In the Algonkian series are quartzites, conglomerates, graywackes, 
slates, dolomites, griinerite-magnetite schists, jaspilites, mica-schists and 
a series of basic tuffs that have suffered metamorphism until they are 
now largely hornblende-biotite-schists. The production of hornblende 
in these tuffs is on a very extensive scale. ‘The amphibole is in large 
plates that are more or less idiomorphic, and nearly all of these are 
cellular in structure. The greenstones that are intrusive in the iron 
formation are altered diabases, in only a few of which augite can still 
be detected. These rocks have heretofore been generally known as 
diorites. In the western part of the district where the squeezing has 
been more intense than elsewhere much quartz and biotite has been 
developed in them, and the rocks present a very different aspect from 
the more normal phases. 
All the rocks mentioned above are described in detail, and many 
colored reproductions of polished hand specimens of limestones, slate 
and jaspilite illustrate the report. 
The Rock-formation of the Silver Cliff and Rosita Hills 
District, Colorado.—The mines of the Silver Creek and Rosita dis- 
trict are mainly in volcanic rocks. The eastern portion of the area is 
the seat of an old voleano which erupted andesite, rhyolites and trach- 
ytes both in lava and in tuff form. ` These constitute the Rosita Hills. 
North and east of these are the old granites and gneisses. In the 
3 Bull, 62 U. S. Geol. Survey. 
