Davis.] 
486 
[May 18, 
The cobbles and pebbles that characterize the head portion of 
the plateaus consist for the greater part of crystalline rocks, 
but slates and occasional conglomerates are also found. These 
are always fairly well rounded, although but few of them can be 
called well water- worn. None have been found showing unmis- 
takable glacial striations. Large boulders, up to eight or ten feet 
in diameter, are found at various levels in the mass of the 
plateau ; many of these now lie at the bottom of the excavations, 
being too large for removal. Like the smaller stones they exhibit 
much variety of composition, coming both from the crystalline 
highlands on the north and northwest, and from the lowland of 
bedded rocks, followed by the Charles .River. 
After examining the sections by which the internal structure is 
revealed, the observer should return to the margin of the plateau 
at its head, and inquire how the materials of which the mass is 
composed could be brought to their present position. All the 
stones and sand that have come from the stratified rocks of the 
Charles River lowland must have been raised at least a hundred feet 
before they could reach the plateau surface ; and all the crystalline 
rocks have been carried across a distance of at least two or three 
miles now occupied by an open lowland. In order not to give 
too ready acceptance to the theory of the glacial origin of the 
plateau, the following reasons may be advanced for rejecting other 
explanations as entirely insufficient and discordant with manifest 
facts. 
8. CONDITIONS OF DEPOSITION OF SAND PLATEAUS. 
The only agents of transportation that need be considered at 
all are the wind, rivers, the sea, and ice. Leaving the latter out 
of account for the moment, the first may be excluded by the 
coarseness of the materials constituting the plateau ; it cannot 
possibly be regarded as a dune. An origin by either river or sea 
action will require that, at the time of construction, the deposit 
must have stretched continuously from its source in the crystalline 
highlands on the north across the Auburndale lowland to its 
present position ; and hence that the lowland must have been ex- 
cavated after the plateau was built. There is nothing inherently 
impossible in this, as far as the general proposition is concerned ; 
but it is embarrassed by certain local objections that soon drive it 
out of consideration, In the first place, the supply of so much 
