iSSg.] Mineralogy and Petrography . 439 
ning and composition plane is a clino-dome. Upon alteration 
it gives rise to the rare mineral webskyite' and tremolite. The 
former is found to be an intermediate product in the passage 
of olivine into serpentine. Its analysis yielded results which 
indicate for it a composition corresponding to H^ (Mg. le) 
S:o, + 2 Aq, that is a hydrated olivine, with part of its mag- 
nesium replaced by hydrogen. Its specific gravity is 1.745. 
The augite of the rock is brown in color, and, like the oHvine, 
gives rise to a peculiar alteration product. This is a green 
garnet with the composition: 
S: Fe.O,, AlO, Ca O Sp. Gr. 
34-95 30.12 1.77 33.29 3-977 
With it is associated helminth. Pseudomorphs of serpentine 
after magnetite are described in the same rock. A pseudo- 
morph of calcite after chrysotile is mentioned as occurring in 
chrysotile veins in a diabase from Amelose, and pseudomorphs 
of the same mineral after the olivine of this diabase are briefly 
alluded to. An interesting addition to the study of jade and 
nephrite has been made in the shape of an article by Messrs. 
F. W. Clarke and G. P. MerrilF on the chemical and microscop- 
ical characteristics of the materials composing some of the in- 
struments on exhibition in the U. S. National Museum. An- 
alyses, specific gravity determinations, and the study of thin 
sections of nineteen specimens lead the authors to the view 
that the jadeite and nephrite objects, which have been gath- 
ered from widely scattered localities, cannot be depended upon 
m the work of tracing the migrations of ancient tribes of peo- 
ple, since (i), the material of which the objects consist is by no 
means always a pure jade or nephrite, and (2), these substances 
themselves, when obtained from different sources possess very 
different characteristics. The analyses and microscopical 
studies yield httle new in regard to the structure of true jade. 
The paper is of importance as affording proof of the existence 
of a true jade in Alaska, and as a record of the results of the 
examination of many jade instruments not heretofore described. 
Near the village of Trevalga^* is a shell of eruptive rock, from 
seventy to a hundred feet thick, interstratified with slates. 
Foliation is highly developed throughout its mass in a direc- 
tion parallel to the cleavage of the slates. In places it is 
^ Cf. A.NfERicAN Naturalist. November 1887, p. 1021. 
