184 The American Geologist. March, 1891 
on the north and south cut in red Triassic sandstone, and stretch- 
ing across it in magnificent sweeping curves are two walled 
1 teaches marked by lines of shingle; an outer one, or bar, only 
1 iroken in two places by small streams which cross it at low tide \ 
an upper one, or normal storm beach, also broken in two places 
but at high as well as low tide. A coastwise stream flowing 
south during the retreat of the tide and north during its advance 
has produced the outer beach aided by the configuration of the 
coast line at this point. In all respects, but two, it is in nowise 
dissimilar to ordinary bars formed southward along the Massa- 
chusetts coast. These two differences are dependent on the ab- 
normal run of the tides in the bay. Here the bar forms an 
outer water line at low tide ; off Massachusetts such a bar marks 
a water line at high tide only. The Quaco bar is covered by the 
advancing and retreating tides twice a day ; the Massachusetts 
bars are seldom or never covered. To the rapid run of the tides 
in the bay the existence of the bar is due, as the energy of the 
breakers seems to spend itself against the beaches mainly at high 
and low tides, and points intermediate between these feel but 
little of the force of the waves as expressed on the two lines of 
shingle. The tides come up the harbor faster than a man can walk, 
pass over the bar without destroying it and as rapidly advance to 
the upper beach line, completely submerging the lower. Conversely 
the same is true of the retreating tide. In cross section the lower 
beach is very variable not only in its several parts but there are 
variations dependent upon the season. Old residents of Quaco 
informed me that they had known it to be nearly destroyed during 
severe storms from the south-east. Landward, in night above 
the gently-sloping surface of the beach, as a whole it is seldom 
over five feet and locally decreases in elevation so that it practi- 
cally disappears. The seaward slope is much steeper and the bar 
appears much more pronounced viewed from this side. At the 
two ends the ridges of shingle come together outlining a rude 
moon-shaped area. Between these two lines it is difficult to find 
large pebbles such as make the crest of the inner or outer beach 
lines. From this it seems probable that the outer zone of shingle 
was either brought there from the north or south and not trans- 
ported seaward by the undertow from the upper beach. 
In color, the sand forming the bottom of the harbor, which has. 
