246 The American Naturalist. [March, 



nerite, according to Messrs. Lane and Sharpless. 1 Its refraction is 1.7. 

 It may easily be distinguished from actinolite by its polysynthetic twin- 

 ning parallel to °o Ps* and by its optical characteristics. It is color- 

 less or pale green or brown, and is only faintly pleochroic. An approx- 

 imately correct analysis gave : 



Si0 2 A1 2 3 Fe 2 3 FeO MgO (NaK) 2 H 2 

 76.32 .56 .99 6.96 12.47 tr. 2.80 

 Fibres of riebeckite or crocidolite have also been discovered by Lane 

 as a secondary growth on the primary hornblende of a syenite from 

 the S. E. \ of Sec. 17. T. 49, R. 25, W. in Michigan. The princi- 

 pal types of olivine and anorthite skeleton crystals in some of the 

 Yesuvian lavas have been well characterized by Rinne' 2 in an article 

 illustrated by thirty-eight figures. The olivine skeletons are elongated 

 parallel to the axis a. Many are twinned, giving rise to various cmsses, 

 in one of which, whose arms intersect at nearly right angles, the twin- 

 ning plane is «o P2, a new twinning law for this mineral. Intergrowths 

 of olivine and plagioclase were noted. The anorthite skeletons often 

 show crystallographic faces in grains no larger than .07 mm. in diam- 

 eter. Mr. Turner 3 gives a brief account of the geology of Mt. 



Diablo, in California, describing incidentally a uralitized diabase con- 

 taining twinned augite, and in some places passing over into a diorite 

 whose hornblende may be secondary, peridotites (lherzolites),pyroxen- 

 ites (websterite) and gabbros, each of which has given rise to serpen- 

 tine. In a supplement to Turner's paper, Dr. Melville records the 

 results of the analysis of these rocks together with those of sandstones, 

 shales and a glaucophane schist from the same region. The composi- 

 tion of the schist is as follows : 



Si0 2 P 2 0. A1 2 3 Fe 2 3 FeO MnO CaO MgO K 2 Na 2 H 2 



47.84 .14 16.88 4.99 5.56 .56 11.15 7.89 .46 3.20 1.98 

 — A paleozoic leucite-rock, consisting of sanidine, augite, nepheline, 

 and leucite, with the accessories anorthoclase, apatite, zircon and mag- 

 netite in a glassy base is mentioned by Chrustschoff 4 from a locality m 

 Russia. The rock is aphanitic and resembles in appearance some of 

 the Hohentwiel phonolites as well microscopically as in microscopic 



^Anur.Jottr. Sci., Dec, 1891, p. 499. 

 '^Neues. Jahrb. f. Min., etc., 1891, II, p. 272. 

 3 Bull. Geol. Soc. Amer., 2, p. 383. 



