490 - The Formation of Cape Cod. . [August, 
streams, and at the last by glacial erosion. Materials have been 
thus gathered and mixed from sources near and remote. Their 
deposition, excepting the Tertiary beds of Martha’s Vineyard, 
appears to have taken place during the Quaternary period, partly 
before the ice-sheet was extended over this region, partly along 
its terminal front in long series of morainic hills bordered on the 
south by sloping plains of gravel and sand, and partly by 
immense floods poured down from the surface of the melting ice- 
fields during their retreat. 
Since the glacial theory of the origin of the drift was brought 
to the attention of geologists by Agassiz, forty years ago, it has 
been closely compared with all the observed facts, and seems to 
afford for them an adequate and complete explanation. It has 
received the highest kind of testimony ‘to its truth in being 
required to explain new discoveries and to answer questions which 
were not thought of when the theory was announced. The gla- 
ciers of Switzerland were found to furrow, scratch and polish the 
bed-rock over which they move, rounding and planing away its 
projecting points, just as the ledges of all northern countries are 
striated, rounded and worn smooth. This is done by the grating 
of boulders and gravel frozen in the bottom of the ice; and the 
` sides of these stones are, of course, planed and striated the same 
as the underlying rock. In the valleys of the Alps are many 
glaciers ten to twenty miles long, and through this whole distance 
rock-fragments are dropped on them from bordering cliffs, brought 
in by tributaries and wrenched from the ledges beneath ; so that 
diverse kinds, some of them derived from its farthest sources, 
become mingled both at the top and bottom of the ice. Corres- 
ponding to this transportation, grinding and mixture of materials 
effected by the glaciers, there was found over the surface of all 
regions which had striated ledges a most remarkable deposit of 
boulders gathered from distant and diverse sources, indiscrimi- 
nately mixed with gravel, sand and clay. Many of these blocks 
and pebbles have their sides worn flat and marked with striæ, and 
were evidently the agents by which the ledges were similarly 
eroded and marked. This deposit, called unmodified drift or till, 
differs from any made by rivers, lakes or the sea, in showing no 
evidence of the assorting and stratifying action of water. To 
account for its formation and for the accompanying stria on the 
i he ledges, it was pi spies that a mantle of solid ice was accumulated 
Saw 
