342 L. V. Pirsson — Phonolitic Rocks from Black Hills. 



would vary some in different parts of the mass. For Caswell's 

 description the reader is referred to the original memoir as it 

 would be too long to quote here. 



The specimen collected by Mr. Leavenworth shows a dense 

 felsitic looking groundmass of a dark gray color with some- 

 what greasy luster, very thickly dotted with white or pale 

 gray feldspar phenocrysts which attain a size of l cm in diam- 

 eter. Sometimes they are columnar on the d axis and some- 

 times short and stout in their habit. They are so thickly 

 crowded as to form a very considerable proportion of the rock 

 mass. Between them lie occasional small black prisms of 

 augite. 



In thin section the microscope discloses the following min- 

 erals : feldspar, pyroxene, titanite, apatite in the first genera- 

 tion and feldspar, nepkelite, cegirine and sodalite (?) in the 

 second. The large feldspar phenocrysts sometimes show a 

 zonal structure which is indeed plainly visible on the hand 

 specimen where any process of alteration has taken place, as 

 on a weathered rock surface — the center and certain zones 

 being more affected than the outer mantle and remaining parts. 

 Generally they are very fresh, showing only an occasional trace 

 of decomposition along a cleavage crack. Between crossed 

 nicols they sometimes reveal the zonal structure but never any 

 appearance of microperthite from intergrown albite lamellae 

 and never any twinning save that according to the Carlsbad 

 law. 



Since the chemical analysis of the rock — given later — had 

 shown such an excess of soda over potash it was inferred that 

 this feldspar might be anorthoclase and an analysis of it was 

 therefore made. 



To obtain material clear glassy fragments of the phenocrysts 

 were first picked by hand, crushed fine enough to pass through 

 a 100 mesh (to the inch) sieve and then washed free from dust 

 and separated by the potassium mercuric iodide solution. 

 They floated readily at 2*614 which would separate any albite 

 (none sank however) and fell at 2*557 which would support 

 any zeolites or decomposed matter. A considerable portion 

 fell at 2*59 and the average specific gravity was between 2*58 

 and 2*59. Seen under the microscope the material consists of 

 clear homogeneous grains. The analysis gave : 



Ratio. 



Si0 2 66*44 1*1073 6 



Al 2 O s 19*12 -1854 1*005 



Fe 2 3 0-56 



CaO tr. 



NaO 7*91 -1274 



K,0 5-10 -0541 f 



H 2 (ign.).__. C-57 



Total 99-70 



