PBOCEEDINGS OF THE NATIONAL MUSEUM. 



643 



Figa. 1- 



APATITES IX HORNBLENDE ANDESITE. 



specimen No. 3«i9T, V. S. N. M. 



in figure 4 the red color is zonal, while the interior is yellowish. Pris- 

 matic sections are all x)lainly pleochroic, being red when the light 

 passes through parallel to the 

 vertical axis and light yellow 

 when at right angles to this 

 axis. A not less interesting- 

 feature is the amount of cor- 

 rosion from the fluid magma 

 which the larger crystals have 

 undergone and which is shown 

 in the figures, especially Nos. 

 1, 2, 3, and 5. From the fact 

 that the apatite is one of the 

 first minerals to separate out, 

 such results are not unex- 

 pected, but, so far as I am 

 aware, have before not been 



observed to the extent here indicated. This is presunia])ly due to the 

 small size of the crystals, as usually occurring. 



The large forms, like figure 7, show a faint cleavage parallel with the 

 prism. 



Intrusive rocks: Lamprophyrs. — From the lower part of the Flathead 

 shales, north of the East Gallatin River. The rocks described below 

 outcrop at the base of the sandy shales that lie just above the basal 

 quartzite of the Flathead formation, as exposed in the hills about one 

 mile north of the East Gallatin Eiver. They have been traced east- 

 ward about three miles from the most western exposure, where they 

 pass beneath the lake beds, but show again where the Flathead shales 

 cross Dry Creek, three miles farther to the northwest. In all these 

 outcrops they hold the same relation to each other. The upper rock 

 is usually from six inches to a foot in thickness, but sometimes thins 

 out to even less than six inches. It lies in close contact with the 

 shales, is dark gray, nearly black, in color, tough, fine grained, and 

 compact, and shows to the unaided eye only occasional small black 

 crystals evidently belonging to a mineral of the pyroxene group, 

 and numerous small reddish amygdules. This is succeeded by, and 

 seems to pass gradually into, a zone«of decomposed material, which 

 carries numerous scales of black mica, and which is traversed in a 

 direction parallel with the sheets by several veins from one to two 

 inches in width of a light pinkish feldspar. The lower or underlying 

 rock, which also seems to pass into this zone of decomposed mate- 

 rial, ai^pears to the unaided eye as a holocrystalline mass composed 

 essentially of elongated light pink feldspars and abundant small, 

 often radiating, folia of black mica. The microscopic and chem- 

 ical iiroperties of this rock are given below. Although the upper 

 and lower rocks belong apparently to two quite distinct types, their 

 constant association, even when in sheets but a few inches thick, is 



