587 
[Saville. 
bowlder in Rockport, Mass. This bowlder has long attracted the 
curiosity of those who wander along the rocky shores of Rock- 
port, although I believe its true significance has never been fully 
understood until Dr. Sanborn happened to discover the ledge 
from which it was detached. This block has been moved by the 
waves in a northwesterly direction, a distance of one hundred feet 
on the moderately ascending smooth ledge shore of the east side 
of Flat Point, one of the two most extreme eastward projections 
of Cape Ann opposite Thacher’s Island. The dimensions of this 
block are about 5X6X13 feet (about 15 feet maximum length) 
having approximately the form of a four-sided prism. It thus 
contains about 390 cubic feet, which at 168 pounds per cubic foot 
would give it a weight at least 32 tons. The base of the block in 
its original position,'' shown by the ledge from which it was broken, 
is about seven feet above the present level of high tide, and its 
base where it now lies is about fifteen feet above high tide. In its 
transportation the block has been turned over, so that what was 
formerly its upper northwest corner when in placets now its lower 
northeast corner, the length of the block both before and after its 
transportation being approximately north to south. 
The parent ledge is northeast of a dike several feet wide which 
runs northwest to southeast and shows a definitely banded condi- 
tion due to differences in its crystalline structure, its central por- 
tion being porphyritic. After being dislodged from the ledge and 
passing south fifteen or twenty feet, far enough to be beyond the 
western side of the nearly vertical wall from which it was removed, 
the progress of the block across the dike and onward was up a 
gently inclined plane, rising about one foot in ten in the direction 
of the movement. This block is readily recognizable as coming 
from the place mentioned by comparison of its north end with the 
ledge from which it was broken, in respect to the form of these 
two corresponding surfaces, the discoloration produced by weather- 
ing, before the block was separated, and by the presence on both 
surfaces of an obliquely intersecting joint plane. This fracture, 
which is fresh in appearance and with almost unworn edges, differs 
from the four lateral faces of the prism, all of which follow joint 
planes. 
At this portion of the shore, there is no barrier to obstruct the 
full force of the waves driven by the frequent northeast storms. 
At such times there is no part of the North shore where the sea 
