620 ON THE GEOLOGY OF THE ISLAND OF AQUIDNECK. 
passed away, but the reélevation was so quick that even these 
faintly traced lines were not worn away. Equally strong is the 
evidence they give concerning the nearness of the time when they 
were made. Although the rock is not the most enduring, haying 
a considerable tendency to shatter, they stand almost as fresh as 
the carvings on Egyptian stones. 
In attempting to measure the influence of the glacial action 
upon this region the first question which arises is whether we are 
to refer the whole of the erosion which is Clearly to be attributed 
to this agent to the action of the ice time which immediately 
preceded the present geological period, or must look upon it 
as having been in part the work of more ancient glacial 
periods. Besides the general argument that there is a great 
improbability in the supposition that there has been but one glacial 
period in the-earth’s history, we have other reasons for believing 
that glacial action has been a constantly recurring element in. the 
successive geological stages. The hypothesis of Mr Croll, which 
up to the present time is the most satisfactory theory that has 
been presented to account for the coming of the ice time, makes the 
change depend entirely upon the alteration in the eccentricity of 
the earth’s orbit, and the change in the place in that orbit when the 
northern and southern hemispheres get their winter and summer 
exposures to the sun. Whether these hypotheses prove well 
founded or not there can be no doubt that the past geological 
record shows us evidences enough of glacial action. Every great 
conglomerate is in itself such evidence ; nothing but a glacial poria 
is competent to produce or transport, into their present poo 
such masses of pebbles as make up the conglomerate which is foun 
in the carboniferous rocks of this island. At almost every pee m 
the earth’s history we have the same sort of accumulations. — T pe 
abound in the primordial, are frequent in the palaeozole 
mesozoic, and numerous in the tertiary. The Roxbury conglome- 
rate found in the town of that name, and over a large se 
the south and west shores of Massachusetts Bay, is 5° mach fi 
the modern drift, that at one point, where it is rather, wee 
herent than usual, the eye does not at first readily detect the 
ference between the glacial beds of the geological age o 
the underlying conglomerate of the primordial time, 50 Ee by : 
the physical connection of these beds which are separa - 
nearly the whole life-bearing section of the earth’s crust. 
