COMPARISON WITH OTHER ROCKS. 258 
An instructive instance is afforded of the considerable mineralogic con- 
trast that two rocks may present, which seems out of all proportion to 
their chemical differences. 
While the evidence is not perhaps demonstrative, it at least suggests 
strongly that these dikes are all differentiation products from a common 
magma. Their clustered distribution and mineralogic similarity, to- 
gether with their regular chemical variation, make out a good case for 
this view ; yet they range from 69 to 52 per cent of silica, from the acidity 
of granites to a basicity approaching that of basalts. 
The striking cases of differentiation recently described by Broégger 
from Predazzo in the Tyrol and by Weed and Pirsson from Montana in 
quite similar rocks are at once called to mind. In the Bearpaw moun- 
tains, for example, Werd and Pirsson describe the differentiation of an 
augite-syenite magma into an exceedingly interesting series of grada- 
tional phases.* While the physical conditions under which rock solid- 
ification took place were quite different, the effects produced were so 
similar that the analyses are appended : 
1, Il. 1001, AVE 
SOMME ea otis ite amc Gaditelnm shenes 68.96 | 68.384 | 52.83 52.81 
INGO oma cope Op £18 ORG OU CC eae ES CTL 15.25 | 15.32 | 18.31 15.66 
W@Ogca 'b'o 6.00. d 6 SIRE EIS Et OO IE eee nea 3.28 1.90 0.34 3.06 
INO) 5 010.0: 610% GC SCION IE een Se eet arma a Sy aete 0.84 6.43 4.76 
IMNMO pn 0:5-0:0:6.6 AICS DIOS AS ec iretee EER neers 0.23 0.07 0.15 | Trace. 
CRO). 2 6:6 001 » SENy ei) Ce Ae eI EN nan 0.76 0.92 Bo f° TodM 
INTO) on. 6.0.6. Otseile Oe a Cie ees Bane acta ea 0.20 0.54 1.82 4.99 
IK; Ooo 6:40.60 6 oi chet AOBIRES Te aT cr eI aces ame eS 5.01 5.62 6.47 4.84 
INE OMe rie rcs) es cristo weiss ae sew ieeie 5.46 0.45 7.26 3.60 
TPQ): 0.0 0 6 ata ec Bbc A Hee OO chee Tat eel are CD 0.13 1.59 0.75 
Ch pond doce inerrant Re SS costs bk gad eat PORE 0.04 0.40 0.07 
1D ooh an Goa RENT Teeter cxepm iueiede oasis ceany eysweeceueyeral| ebayenego cs 0.32 | Trace. 
JL@SS « 0.6.01 3.0'8 "6 roadke ct erect pe ec ee eRe ee ea 0.91 0.45 1.16 1.09 
100.25 | 99.95 |} 99.93] 100.24 
I. Dike 29, Clinton county, New York. 
II. Quartz-syenite, Beaver Creek stock, Bearpaw mountains, Montana. Analysis 
by H. N. Stokes. American Journal of Science, May, 1896, page 354, analysis I. 
II. Dike 9, Clinton county, New York. 
IV. Mouzonite, Beaver creek, Bearpaw mountains. Analysis by H. N. Stokes. 
American Journal of Science, May, 1896, page 357, analysis I. 
The Montana rocks show a still further differentiation to the more basic 
type, shonkinite. Nosimilar rock has yet turned up in the Adirondacks, 
and there is strong probability that the Adirondack magma, before dif- 
* Am. Jour. Sci., May, 1896, pp. 351-362. 
