234 ELEMENTARY GEOLOGY 
ever, the main effect of these organisms is to pro- 
tect the shore which they cover with a veritable 
mat, against which the energy of the wave attack 
is expended. 
Destruction of the Coast. — The amount of change 
produced by these oceanic agents cannot be stated, 
because it varies so much from place to place. On the 
hard, rocky coast of New England, no noticeable change 
has been made in the 250 years and more since the 
Pilgrims came over; yet this is exposed to the action of 
violent ocean waves. The cliffs on eastern Martha’s 
Vineyard, on the other hand, have been worn at a 
perceptible rate. The Nashaquitsa Cliffs of this coast 
have been cut back a distance of 220 feet between 
the years 1846-1886, or at an average rate of five 
and one-half feet per year. On the coast of England 
there are many places where’ the shore line has moved 
inland at a measurable rate, in some localities as 
rapidly as a yard per year. 
In such places bars and beaches are also growing as 
the cliffs are worn back, the materials derived from 
the cliffs being partly built into the beach. The 
amount of shore modification which has been recorded 
in various parts of the world, warrants the belief that 
the coast line is a scene of constant change, and that 
in. the lapse of ages, wave attack and tidal action 
have materially altered our sea-coasts. 
