F, L. Ransome — Apatitic Minette. 337 



Art. XXXYIII. — An Apatitic Minette from Northeastern 

 Washington y* by Frederick Leslie Ransome. 



In the course of a rapid reconnaissance along the Northwest 

 Boundary in the summer of 1901, numerous dark micaceous 

 dikes were noted on both sides of the Columbia River, between 

 the smelter-town of Northport, Wash., and the settlement of 

 Waneta, at the mouth of Pend d' Oreille River, in British 

 Columbia. These dikes cut a series of sediments of unknown 

 age, possibly Carboniferous, consisting of steeply inclined dark 

 slates, banded schistose limestones, and subordinate quartzites. 

 Similar dikes were noticed for 12 miles eastward along the 

 Fend d' Oreille and as far west as Kettle River, which crosses 

 the boundary about 40 miles from Waneta. Not all of these 

 dikes have been carefully studied, but most of them appear to 

 be minettes or rocks closely related to this type. One speci- 

 men from a particularly micaceous dike at the mouth of Sheep 

 Creek, on the northwest bank of the Columbia opposite North- 

 port, has been analyzed by Dr. W. F. Hillebrand and found 

 to be of rather unusual chemical character — enough so, it is 

 thought, to warrant the publication of the analysis with a 

 petrographical description. 



The rock is dark, greenish gray, by far the most prominent 

 constituent in hand specimens being the closely crowded scales 

 of biotite up to 4 or 5 millimeters in diameter. The other 

 mineral constituent's form in general a fine-grained matrix to 

 the biotite and are not individually distinguishable without a 

 lens. Parts of the rock, however, contain irregular light- 

 colored streaks in which the feldspar is a little more distinct 

 than elsewhere. 



Under the microscope the rock shows a poikilitic texture, 

 large irregular areas of optically continuous feldspar being 

 crowded with automorphic crystals of biotite, pyroxene, apa- 

 tite and titanite. The feldspar is chiefly orthoclase, although 

 much of it is not optically homogeneous and some evidently 

 contains microperthitically intergrown albite. It is all more 

 or less turbid with minute, brown, dust-like inclusions. The 

 biotite is chestnut-brown in most sections with the usual 

 strong absorption parallel with the cleavage. The axial angle 

 is small and the interference figure shows no distinct opening 

 of the lemniscate cross into hyperbolas. The pyroxene, which 

 is automorphic in the prism zone with occasional terminal 

 planes, is monoclinic with a maximum observed extinction of 

 ZaC of about 45°. It is colorless to very pale green, non- 

 pleochroic and is probably an augite near diopside in compo- 



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



