164 F, BASCOM 
abundant augite and hornblende.* A further justification of 
placing in the syenite group a rock showing so large an amount 
of the ferro-magnesian constituents is found in the new group 
of rocks recently described by Weed and Pirsson from the 
Highwood Mountains? and Yogo Peak,3 Montana, in the lam- 
prophyres of Gallatin, Jefferson and Madison counties, Mon- 
tana, described by Merrill,# and in the banakites of the Yellow- 
stone National Park and vicinity described by Iddings.® 
Here we have a series of rocks with orthoclastic ground- 
masses combined with variable proportions of the ferro-magne- 
sian silicates (augite, hornblende, biotite and olivine) with a 
wide silica range, all related to the syenite group. 
Age.—That the drift material in the neighborhood of Columbus 
has its source from the north shore of Lake Huron in Canada 
and from northern Michigan was sufficiently indicated by petro- 
graphic study. Distinct types from the bowlders were matched 
both in the hand specimen and under the microscope to speci- 
mens taken from ledges in Canada and Michigan. While such 
matching was not valuable with the monotonous granites, 
gneisses and quartzites which constitute the great bulk of the 
bowlder material, a sufficient number of pronounced types were 
harmonized to establish both the age and the general source of 
the bowlder material. 
Representatives of the Archean, Algonkian, Cambrian 
and Lower Silurian formations are found among the bowlders. 
All the igneous material is undoubtedly pre-Cambrian or Cam- 
brian. The presence of this member of the nepheline-syenite- 
porphyry group cannot be explained except on the supposition 
that it was, like the material among which it was found, brought 
=W.C. BROGGER, Die Eruptivgesteine des Kristianiagebietes. I. Die Gesteine 
der Grorudit-Tinguait-Serie. Kristiania, 1894. 
2 Highwood Mountains of Montana, Bull. Geol. Soc. Amer., Vol. VI., p. 414. 
3Igneous Rocks of Yogo Peak, Amer. Jour. Sci., Vol. L., No. 300, Dec. 1895, 
pp. 467-479. 
4GEORGE P. MERRILL, Notes on Some Eruptive Rocks from Gallatin, Jefferson 
and Madison counties, Montana. Proc. U. 5. Nat. Museum., Vol. XVII., pp. 665. 
5]. P. IppinGs, Absarokite-Shoshonite-Banakite Series, THE JOURNAL OF GEOL- 
ocy, Vol. III., No. 8, Nov.—Dec. 1895, pp. 935-939. 
