24 PHILOSOPHICAL SOCIETY OF WASHINGTON. 
ec, dd’, ete., will produce a parting parallel to the arc of the circle 
cd’ é,etc. If, however, the resistance in a direction parallel to ae 
be neutralized by some force, the resistance along the different radii 
will be diminished by the amount of the vertical component in each 
case, and the resulting fracture will be parallel to the ellipse ¢ 0’. 
The relative tendency to fracture also is represented by the area 
bcd’. 
Such a parting is actually present in the large columns in John 
O’Rourke’s quarry, the major axis of the ellipsoid being vertical, as 
it should he if the weight of the superincumbent mass counteracted 
any resistance to contraction in a vertical direction. 
The wavy form of the columns, large and small, suggests irregu- 
larities in the mass which disturbed the uniform advance of the lines 
of maximum strain and caused them to deviate from parallelism. 
The superficial banding of the large vertical columns by nearly 
horizontal notches or grooves, resembling layers of bricks or rude 
chiseling, appears to be simply a modification of the plane of the 
crack. 
The paper closes with a description of the microscopical charae- 
ter of the Orange rock, which from its identity with many recent 
basalt flows leads the writer to the conclusion that it should be 
classed as a coarse-grained basalt or dolerite, as Prof. E. S. Dna 
has called the similar rocks in the Connecticut valley. 
The occurrence of the rock in question as a surface flow is ren- 
dered highly probable by its glassy nature, and the disposition of 
the columns, which resembles that of many lava sheets in western 
America, as well as of those in central France. G. Poulett Scrope, 
in his work on “ Volcanos” (2d edition, London, 1872), discusses 
the question of the origin and nature of columnar structure in 
lavas and other substances, and by a somewhat different course of 
reasoning arrives at essentially the same conclusions as those reached 
in the present paper. 
Mr. W. J. McGeEsr then made a communication on 
THE TERRACES OF THE POTOMAC VALLEY. 
Remarks were made by Messrs. Warp, Toner, Ropinson and 
Bates. 
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