356 MR. TATE ON POLISHED AND SCRATCHED ROCKS, 
on a submarine hill, the force with which it will move over the 
surface beneath, and the gravity with which it will press on that 
surface will be amply sufficient to polish and striate any rock 
however hard. 
Another modification of the same agency has probably played 
a still more important part in the ancient sub-arctic sea. The 
coasts of Scandinavia, Nova Scotia, and more northern regions 
are bound by ice of considerable thickness ; the shingle and mud 
upon the beach are entangled in this ice ; rocks fall upon it from 
the coast cliffs, and when it is broken up, separate masses are floated 
away by marine currents and tides, carrying with them the debris 
of the coast, and as they are driven along the shore, they polish 
and scratch the rocks with which they come in contact. Where 
the rocks, embedded in the boulder formation, exhibit polishings 
and scratchings, and are of local origin, the phenomena may be 
best accounted for by this kind of agency. 
To attribute, however, too much to any single agency would 
very imperfectly explain the varied phenomena connected with 
the Northumberland Boulder formation. They are the result of 
the complex action of several agencies. Viewing the whole of 
the facts, and deriving additional evidence from other districts, 
I am led to conclude that the boulder formation era, in North- 
umberland, extended over a long period of time, during which 
the climate was of a sub-arctic character, and that the whole 
of the county was under the sea excepting the higher hills, 
which would form islands in the midst of the waters; the 
tides and currents, acting on the shores and sea_ bottoms, 
broke up the rocks, reducing to small particles the softer 
sandstones and shales, and redistributing them as beds of sand 
and clay; the harder limestones and porphyries rounded by 
long attrition on each other, and eventually arrested by inequali- 
ties of the sea bottom, would be irregularly heaped up in gravel 
beds: strong tides and currents, sweeping along with increased 
fury by gales of wind, may have detached larger masses of rock 
from their native beds, and these after being rolled about, may 
have been plunged into and retained by the stiff clay in which 
they are usually found ; the diversified nature of the coasts 
