THE POPULAR SCIENCE MONTHLY. 



edge, but that much more remains still to 

 be determined. It seems as likely that in 

 this problem geology and meteorology will 

 - the word of command to physics as 

 the converse. At present our knowledge of 

 a definite limit to geological time has so 

 little precision that we should do wrong to 

 reject summarily any theories which appear 

 to demand longer periods of time than those 

 which now seem allowable. In each branch 

 of science hypothesis forms the nucleus for 

 the aggregation of observation, and as long 

 ;:- facts are assimilated and co-ordinated we 

 ought to follow our theory. Thus, even if 

 there be some inconsistencies with a neigh- 

 boring science, we may be justified in siill 

 holding to a theory, in the hope that further 

 knowledge may enable us to remove the dif- 

 ficulties. There is no criterion as to what 

 degree of inconsistency should compel us to 

 give up a theory, and it should be borne in 

 mind that many views have been utterly 

 condemned 'when later knowledge has only 

 shown us that in them we were only seeing 

 the truth from another side. 



An Inventory of the Glacial Drift. — 



Vice-President Chamberlin, in his address 

 before the American Association's section 

 of Geology and Geography, presented "An 

 Inventory of our Glacial Drift." Having 

 described the boundaries of the drift as 

 represented on a wall-map, the speaker re- 

 marked that a wealth of significance lay in 

 the sinuosities, vertical undulations, and va- 

 rying characters of the southern border. It 

 undulates over the face of the land essen- 

 tially much as an arbitrary line from New 

 York Harbor to Puget Sound, and could be 

 reduced to horizontally — as it must have 

 been to have marked the margin of some 

 ancient ice-bearing body of water — only by 

 incredible warpings and dislocations. The 

 border presents three notable phases : one 

 part terminating in a thickened belt, a ter- 

 minal moraine; another in a thin margin; 

 and a third in an attenuated border of scat- 

 tered pebbles. The morainic border pre- 

 vails in the Atlantic region and on or near 

 the limit as far west as Central Ohio. 

 Throughout the rest of the stretch to the 

 Rocky Mountains the attenuated edges pre- 

 vail. Of mist ratified bowldery clays or tills, 

 there is the richest variety, ranging through 



diverse combinations of material, texture, 

 and aggregation. Of moraines, terminal, 

 lateral, medial, and intermediate varieties 

 are found. The great terminal moraines 

 overshadow all others in interest and im- 

 portance. Outside of the chief moraines 

 are occasional belts of older drift aggre- 

 gated in the similitude of peripheral mo- 

 raines. Back from the two principal ter- 

 minal moraines lie similar partially deter- 

 mined belts, usually of less prominence and 

 continuity. Our most unique moraines are 

 the interlobatc, developed between the 

 tongues into which the ice-sheet of the sec- 

 ond epoch was divided at its margin, of 

 which about a dozen, in half as many States, 

 are recognized. Beautiful lateral moraines 

 abound in the mountainous regions of the 

 West, and some were developed by local 

 glaciation supervening upon the ice retreat 

 of the East. Our medial moraines are unim- 

 portant, and confined essentially to mount- 

 ainous glaciation. Allied to the true mo- 

 raines are special forms of aggregation of 

 the sub-glacial debris. Two classes com- 

 monly embraced in the assorted drifts should 

 be excluded from them : the " orange sands " 

 of the Mississippi Valley, which do not ap- 

 pear to possess the distinctive characteris- 

 tics of glacial gravels, but are residuary in 

 aspect ; and the secondary drifts, or those 

 that have been reworked by wholly ncn- 

 glacial agencies. Eliminating these, two 

 classes of products of glacial waters work- 

 ing co-ordinately with the ice are recog- 

 nized : those that gathered immediately with- 

 in and beneath the ice-body itself, or against 

 its margin; and those which were borne to 

 distances beyond its limit by the glacial 

 drainage or by peripheral waters. The prod- 

 ucts embrace a great variety of sub-types 

 of gravel - heapings, including isolated 

 mounds, conical peaks, clustered hummocks 

 with inclosed pits and basins, and sharp, 

 steep - sided ridges, often of phenomenal 

 Length, all possessing great irregularities of 

 material and stratification, embracing fre- 

 quently, manifest disturbances. The elon- 

 gated variety, resembling the great omrs of 

 Sweden, arc finely developed in Eastern New 

 England ; while the hummocky variety, con- 

 stituting the ill-defined class of kames, arc 

 abundant throughout New England, New 

 York, Northern New Jersey, Pennsylvania, 



