
236 THE AMERICAN NATURALIST. [VoL. XXXVI. 
pyroxenes and amphiboles. The group of the perknites would include 
pyroxenites, hornblendites, and hornblende-pyroxene aggregates. it 
would include also websterites and similar aggregates in which mono- 
clinic amphiboloids predominate. The augitites and. some of the 
hornblende-picrites are the corresponding effusive types. Analyses 
of some of the author’s characteristic perknites follow : 
SiO, AlO; FeO; FeO MgO CaO Na,O K,0  H,O Various 
2 , ` . 
I. 46.28 19:54 9.91 2.21 1.89 : 
II. 48.04 7.82 2.01 9.32 13.33 13.01 .69 -48 3.07 2.74== 99.8t 
HI.. 48.63 8.33 2.91 3-90 21.79 13.04 db 23 2.81. ¥,46== 700. 13 
IV. 50.80 3-40 1.39 8.11 257 12.31 tr tr .52 +73 = 100.03 
M... 83.25 2.80 69 5-93 19.9! 16,22 +19 tr 29° * o>. 09.98 
Viv) 53:25 1.04 1.44 7.92 20,78 13.12 JI :07 1,01 .87 = 100.47 
I. Hornblendite. Dyke near Silver Peak Village, Esmeralda County, Cal. 
II. Amphibole-pyroxene rock. Abundant in Mariposa County, Cal. 
III. Perknite (peridotite). Belchertown, Mass. 
IV. Pyroxenite. Johnny Cake Road, Baltimore, Md. 
V. Websterite. Mt. Diablo, Cal. 
VI. Websterite. Oakwood, Cecil County, Md. 
Shonkin Sag; A Study in Differentiation. — Shonkin Sag is a 
flat laccolite rising from the plains southeast of the Highwood Moun- 
tains, Montana. It has afforded Weed and Pirsson ! excellent oppor- 
tunities for the study of the differentiation of a magma in place, since 
it is dissected by a stream valley formerly occupied by a branch of 
the Missouri River. Examination of the sections exposed to view 
indicate that the igneous portion of the laccolite consists of concen- 
tric shells of leucite-basalt, skonkinite, a transition rock composed 
of groups of large augite crystals and long slender foils of biotite 
with white feldspathic material between, and syenite, with the last- 
named rock in the center. The various rock types grade into one 
another without discernible breaks between them, ‘The facts suggest 
to the authors “that the body of the magma forming the laccolite 
must have been injected as a whole, in a homogeneous condition, 
and that the rearrangement and formation of the various parts fol- 
lowed within the mass itself.... The first stages of cooling’ and 
crystallization against the outer envelope of sedimentary rocks was 
relatively more rapid than that affecting the inner portion, and 
resulted in producing the outer porphyritic shell.” There was, 
further, a gradual withdrawal and concentration of feldspathic mate- 
rial toward the inner portion of the mass and an enrichment of the 
outer zone with lime, iron, ‘and ‘magnesia, thus producing the concen- 
c dro. 

