Parallel and Intersecting -hunts. — Crosby. 371 
and in this direction lies the most serious objection to the tor- 
sion theory, the contrast between the two theories here being 
similar to that between the earthquake theory and the old 
contraction hypothesis. Strains developed under the condi- 
tions that obtain in the earth's crust, by simple plication, tor- 
sion, or shrinkage, must, in the nature of tlu- case, approach 
the breaking point so gradually that, unless the rock is of an 
extremely homogeneous character, the yielding by rupture 
will be distributed over a considerable period of time. The 
rock gives way at the weakest point first, and throughout the 
breaking is influenced by every detail of texture and structure, 
the final result being far more irregular than under the instan- 
taneous and alternating stresses that mark the passage of an 
earthquake. 
In all cases, therefore, where the joint-structure of the 
rocks exhibits a high degree of regularity, we may fairly re- 
gard the torsion theory, at least in its present form, as inap- 
plicable, as not affording an adequate explanation. This 
conclusion is particularly safe in those cases where jointing 
of exceptional regularity is observed in rocks of coarse and 
irregular texture. It is certainly well-nigh inconceivable 
that the remarkably plane and regular jointing of the Rox- 
bury puddingstone and the coarse Newport conglomerate, in 
which the hard pebbles of quartzite, felsite and granite, and 
the yielding matrix, have been neatly divided as by one 
swift stroke of some Titan's sword, can be the product of 
slowly developed torsional strains. The torsion theory also 
fails most signally to explain the very perfect and regular 
parallel and intersecting joints sometimes observed in uncon- 
solidated deposits, as in the Miocene clays of Virginia* and 
even in the glacial clays of New England. 
The preceding paragraphs may be summarized as follows: 
A single system of joints, dividing the rock into sheets but 
not into blocks, appears to be best explained by earthquake 
vibrations, or by simple plication if not of a high degree of 
regularity. Two systems of unlike character are also best ex- 
plained by the earthquake theory: but two systems of similar 
character may be referred to the torsion theory, unless, again, 
they exhibit the regularity indicative of instantaneous stresses ; 
*Proc. Boston Society of Natural History, xxin, 245. 
