Origin of Plienocrysts. — Crosby. 299 
Review of the Glacial History. 
All the drift formations described in this paper belong to 
the closing Wisconsin stage of the Glacial period. 
Overlapping and interstratification of till from the north- 
east and the northwest were due to fluctuations of the belt of 
•confluence of opposing glacial currents. 
When the final recession of the ice-sheet extended to this 
vicinity, the area of the glacial lake Hamline, in St. Paul, was 
the first part of Ramsey and Hennepin counties to have its 
ice envelope melted through. 
From the west side of lake Hamline, the ice-sheet upon 
Minneapolis receded westward, terminating along north to 
south boundaries, with the formation of marginal moraines 
which lie on the eastern and western limits of the city. 
The modified drift plains and eskers of Minneapolis were 
the work of glacial rills, brooks, and rivers, pouring down 
from courses on the ice surface, where the melting had expos- 
ed much previously englacial drift. 
Deposition of stratified gravel and sand inclosing lake ba- 
sins, and forming a chain of lakes in the southwest part of the 
city, was caused to take such deeply hollowed forms by the 
persistence of masses of the ice left while the main ice bound- 
ary was withdrawn. 
ON THE ORIGIN OF PHENOCRYSTS, AND THE 
DEVELOPMENT OF THE PORPHYRITIC 
TEXTURE IN IGNEOUS ROCKS, 
By W. O. Crosby, Boston. Mass. 
The ideas embodied in this paper were, in the main, elab- 
orated in the spring of 1898, in the course of a correspondence 
with a fellow geologist upon the geological significance of .the 
prophyritic structure of the porphyritic aporhyolyte or quartz 
porphyry forming the contact zone of the granite batholith of 
the Blue hills, south of Boston; but in part, also, during an ex- 
cursion in the following summer, to the head waters of 
the San Joaquin river, in California. When, in the following 
December, at the New York meeting of the Geological Soci- 
