200 The American Geolofjisl. Marcli,i8y5 
Prof. J. W. SpKNCKK, disciissiiij,'' tliis pa|)rr. jiltribiiti'd tlic hiuli Icr- 
races to di'ixisilioii at tlif sea Icxel, wlieii Ni'W l^nuland was drprcssfd 
1,000 to l..")()0 fcft: and hi' tliouii'lit the maximutn siibsidiMicr indicated 
in thi' Wliile inomilaiiis lo he jibout 2,000 tVci. 
]'(iriationx i/f (rlnrirrs. \l\ IfAliiiY Fiem)IN(i Rkid. (ircat iidcrcst 
has breii aroused hitely in tlie study of the variations of glaciers. Ob- 
servations on tlie lilaeiers of tiie Ali)S liave siiovvii that tlieir diniensions 
iinderjro a |)eriodic chaiiiic: that tliey inerease. attain a rr.axiniiini, de- 
crease, reacli a minimum, aiul begin to increase iigain, jioini;- Ihrougii 
such a cycle two or tliriM' times in a centui-y. Records of more or less 
exactness extend baclv two or three liundred years. (Jlaciers, liowever. 
are i;ot all in the same [)hase at the same time; indeed, some bej4in to 
advance when others liave almost reached their maximum. Glaciers 
side by side are sometimes found in oi)posite phases. This makes it 
difficult to determine the relation between variations of climate and of 
irlaciers,but some progress h;is been made. The theories of Ricliter and of 
Forel, advanced to explain this [x'culiar behavior of jil'iL'iers, were pre- 
sented. Accurate and extended information is wanted concerning' the 
clianges that glaciers undergo. At the International Congress of (ieol- 
ogists in Zurich last suTumer, a committee was appointeil to colled in- 
formation on this subject all over tlie world; and the author, wdio is 
one of this committee, desires notes from all American observers (ad- 
dress: Johns Hoi)l<ins I'niversity, J^altimore, Md.). The remainder of 
the pai)er gives methods, from the simplest to the more ditticult, of ob- 
serving and recording tlie extent of glaciers. (This paper is to be pub- 
lished in the Journal of Geology.) 
DLseviiiuiKition of Glacial Accumulation and Incadon. ]jy Wauhen 
Upham. The accumulation of ice-sheets by snowfall on their entire 
area is discriminated from an advance or invasion by the front of the 
ice, extending it thus over new territory. The former condition is 
shown to have been generally prevalent, on the glaciated portions of 
both Morth America and Europe, \)\ the occurrence of comparatively 
small areas of ice accumulation beyond the extreme boundaries of the 
principal ice-sheets. The latter condition, or ice invasion, is indicated 
on tlie outer part of the drift-bearing area eastward from Salamanca, N, 
Y., through Staten and Ijong Islands, Martha's Vineyard, and Nan- 
tucket, where the soft strata beneath tlie ice were dislocated and folded. 
Near the margin of drift-bearing areas, glaciation chiefly due to snow 
and ice accumulation, with less supply by inflow from central and 
thicker tracts of the ice-sheet, is indicated, as the author thinks. l)y a 
gradual att(Miuation of the drift, absence of nioi'ainic knolls and hills. 
and scanty glacial erosion of the bed-rocks. 
Conversely, the evidences of an invasion of Ihe ice-slu'et upon its 
marginal tracts consist in thick drift dejiosits. hilly moraine Ijells, and 
much planing and striation of the rock surface. Disiilacement and 
folding of soft strata beneath morainic deposits seem to be especially 
conclusive proof of vigorous incursion by a steep ice-front. 
Readvances of the ice within its drift area may be recognized by very 
clearly defined Ijelts of morainic hills, jmshed out upon smooth tracts 
of till, or by the very rare occurrence of disrupted or dee]ily crumpled 
underlying l)eds. In most cases where moraines belonuing to tlie time 
of general glacial recession are conspicuously hilly on I heir outer side. 
they record some glacial reiidvance. rather than a mere jiali or a slack- 
ening in the rate of retreat. 
Climatic Coiiditioas .sho/ni hif Xarlh Aiiicrirtni intcrylacial dcpoxitN. Ry 
WAiiHKX Ui'ii.\,M. During the times both of general accumulation and 
growth of the ice-sheeis and of their final recession, fluctuations of 
their borders were recorded in \arious disli-icls by forest trees, jieal. and 
