1884.) The Genesis of Crystalline Rocks. 605 
were in process of final liquefaction. Certain elements of the 
geological situation recently developed by American scientists, 
and already touched upon in this paper, necessarily enter as es- 
sential factors into the solution of the problem before us. The 
following is a summarized recapitulation of glacial principles 
enunciated by authorities now engaged in deciphering the ice 
chronicles of the Northwest: 
That this whole region has been subjected to a period or peri- 
ods of glaciation antedating the last. 
That a longer or shorter era of deglaciation, known to scien- 
tists as an interglacial epoch, intervened between the deposit of 
the older tills and the readvance of the northeast and northwest 
glaciers during the latest ice period. 
That the general direction of glaciation was similar during the 
tarlier and later ice epochs, though it was not always precisely 
the same, 
That glaciation was far more extended in its range at the for- 
wer of these periods than at the latter; the ice of the former 
having Stretched fully three hundred miles south of the termini 
of the modern morainic lobes. 
That the modern morainic belt defines the limit of extension of 
the glaciers during the last ice epoch. 
Finally, that the Little Falls quartz stratum lies between the 
= Cmm moraines of the northeast and northwest glaciers, in a 
medial _morainic belt which is characterized by a system of 
overlapping tills, as described in Mr. Upham’s paper at the 
of this article, 
ae above elements sum up the glacial situation as understood 
Specialists, so far as concerns the quartz stratum under con- 
| sideration, The tills of Morrison county have, up to the present 
SARR been fully reported upon, and their special interrelations 
accordingly very imperfectly understood. 
(To be continued.) 
—— 0: 
THE GENESIS OF CRYSTALLINE ROCKS? 
ee BY T. STERRY HUNT, LL.D., F.R.S. 
| g Writer began by an account of the various hypotheses 
Shans Proposed to explain the origin of the ancient crys- 
mine Stratified rocks, of which gneiss may be taken as the type, 
Oe of a paper read before The National Academy of Sciences, April 15, 1884. 
