368 The American Geologist. December, 1893 
on any reliable facts of thickness, extent and character of strata, a 
result that does not pass below 2~> to 30 million years as a minimum and 
(50 to 70 million years as a maximum for post-Archean geologic time. 
I have not referred to the rate of development of life, as that is 
virtually controlled by conditions of environment. 
In conclusion, geologic time is of great but not of indefinite dura- 
tion. I believe that it can be measured by tens of millions, hut not 
by single millions or hundreds of millions of years. 
Description of Map. 
On the map the hypothetical areas of the Cordilleran, Mississippian 
and Appalachian seas are clearly indicated. The land area west of the 
Cordilleran sea is numbered No. 1, and the Californian sea and the area 
of Paleozoic deposits of western British Columbia, No. 10. The north- 
ern extension of the Cordilleran sea (No. 9) is continued as the Pale- 
ozoic-Devonian sea to the Arctic ocean. The early Cambrian land area 
(No. 2) east of the Cordilleran sea must have been more or less covered 
by water during later Paleozoic time. The area now covered by 
Mesozoic deposits, indicated by No. 3, was presumably covered by the 
westward and northward extension of the Paleozoic-Mississippian sea. 
The area east of the Appalachian sea is indicated by No. 4; and the 
supposed land barrier between the Hudson Bay and the Mississippian sea 
by No. 6; it is not improbable that during Ordovician or Silurian 
time a sea may have connected the two latter seas. The region to the 
south, indicated by No. 5, is supposed to have been covered by the 
southward extension of the Appalachian, Mississippian and Cordilleran 
seas. It is now covered by deposits of Mesozoic and Cenozoic age. 
A more detailed description of the map can be gained from the sec- 
tion on the growth of the continent and on the geographic conditions 
accompanying the different depositions of Paleozoic sediments in 
the Cordilleran sea. 
THE ORIGIN OF PARALLEL AND INTERSECTING 
JOINTS. 
By W. O. Crosby, Boston. Mass. 
Since the general abandonment of the contraction hypothe- 
sis as an adequate explanation of parallel and intersecting 
joints, two theories have claimed the attention of geologists: 
the torsion theory, so ably presented by Daubree,** and the 
earthquake theory, proposed eleven years ago by the present 
writer. f The torsion theory is strongly supported by experi- 
ment, for the results obtained are certainly strikingly anal- 
ogous to the phenomena observed in the natural ledges; and I 
*Geologie Experimental, pp. 300-374. 
tProc. Boston Society of Natural History, xxn, 72-85. 
