ESTIMATES AND CAUSES OF CRUSTAL SHORTENING. 1 



INTRODUCTORY. 



In the following paper I shall use the words crust and 

 nucleus as terms by which to conveniently refer to the outer 

 known solid shell of the earth, of which we have direct knowl- 

 edge, and to the core surrounded by the crust, of which we have 

 only inferred knowledge. The use of these terms in this sense 

 is independent of any hypothesis as to a sharp boundary between 

 the two, and of any theory as to the condition of the interior of 

 the earth. So far as my present purposes are concerned, the 

 nucleus may be entirely liquid, entirely solid, part liquid and 

 part solid, or in a state of matter of which we have no 

 knowledge. 



The intricate phenomena of earth deformation, and particu- 

 larly that form of deformation called folding, has led geologists 

 to assume, in order to account for the facts in the field, that the 

 surface of the earth must have been vastly shortened during 

 geological time. Some instances of the estimates of the amount 

 of crustal shortening may be mentioned. 



Dutton 2 thinks, to explain the phenomena of folding since 

 the close of the Cretaceous, that the radius of the earth must 

 have been shortened more than thirty miles. He states that the 

 plications are so great that we must assume a contraction on 

 some circles of latitude since the commencement of the Permian 

 amounting to many hundreds of miles, and this amount of con- 

 traction is small, he says, compared with that involved in the 

 Laurentian rocks. Heim 3 estimates the transverse shortening of 



1 Published by permission of the Director of the U. S. Geological Survey, 



2 A criticism upon the contractional hypothesis, by C. E. Dutton : Am. Journ. 

 Sci., Vol. VIII, 1874, P- 121. 



^Mechanismus der Gebirgsbildung, von ALBERT Heim : Basel, Band II, 1878, S. 

 213. 



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