JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. Ill OCTOBER 19, 1913 No. 17 



PETROLOGY. —Magnetite basalt from North Park, Colorado.^ 

 H. S. Washington and E. S. Larsen. Geophysical Labo- 

 ratory and Geological Survey. 



The rock described in this paper was collected by A. L. Beekley 

 of the United States Geological Survey, to whom the authors 

 are indebted for the material and for the description of its occur- 

 rence. The specimen was collected at the top of Pole Mountain 

 in North Park, Colorado, about three miles southwest of Coal- 

 mont. The summit of Pole ]\Iountain is a small remnant of the 

 Tertiary volcanic rocks which are extensively developed on the 

 higher slopes to the south. The outcrops are poor and the mate- 

 rial probably represents a breccia. Most of the neighboring Ter- 

 tiary volcanic rocks are basaltic; but they are rich in soda and 

 in them magnetite is not exceptionally high. 



The magnetite basalt is black, aphanitic and very porous; 

 megascopically it shows no determinable crystals. The micro- 

 scope shows that it is holocrystalline, and is made up of about 

 one-half magnetite, with nearly equal amounts of bytownite and 

 pale green augitic pyroxene, and a smaller amount of apatite. 

 The texture is poikilitic; euhedral crystals of magnetite are rather 

 evenly included in anhedrons of both feldspar and pyroxene. 

 The rock is apparently perfectly fresh. On account of the poros- 

 ity no determination of the specific gravity was made. 



A chemical analysis was made, using the standard methods. 

 All of the major constituents, (except the alkalies,) and also 

 titanium were determined in duplicate, with closely agreeing 



^ Published by permission of the Director of the U. S. Geological Survey. 



449 



