ESTIMATES AND CAUSES OF CRUSTAL SHORTENING 5 I 



the Lake Superior region, in western Massachusetts, and in 

 Great Britain, slaty or schistose structures parallel to the intru- 

 sives are common. These structures are conclusive evidence of 

 lateral compression and vertical extension of the rocks intruded. 



As already noted, the whole of the enormous mass of the 

 intrusives and extrusives is to be subtracted from the mass of 

 the nucleus and added to the mass of the shell. Of these two 

 effects the expansion of the crust is without doubt by far the 

 more important. If the nucleus of the earth be taken as having 

 a radius of 3900 or more miles, radial contraction of one mile 

 would involve a loss of volume of more than 190,000,000 cubic 

 miles. A contraction of the radius of the earth of one mile, that 

 is, from 3958.8 to 3957.8, would give a surficial lessening of only 

 100,000 square miles. In the supposed case of nucleal contrac- 

 tion of the radius by one mile, the 190,000,000 cubic miles of 

 material would be available for additions to the crust. If it be 

 supposed through geological time that this amount of material 

 has been uniformly intruded within the outer ten miles of the 

 crust of the earth, this would demand a surface space of 

 19,000,000 square miles, or about one-tenth of the earth's sur- 

 face. As a consequence the material previously occupying this 

 outer shell would be crushed so as to occupy nine-tenths of its 

 original space, and this would involve enormous lateral crustal 

 corrugation, with consequent thickening of the outer shell from 

 ten miles to about eleven miles. 



If it be supposed that the transfer through geological time 

 from the nucleus to the outer five miles of the crust has been 

 only one-tenth of the amount suggested in the above paragraph, 

 the effect would still be great. Under this supposition, the 

 radius of the nucleus of the earth as a result of igneous intru- 

 sions has contracted one-tenth of a mile, and as a consequence 

 its surface has been lessened by about 10,000 square miles. This 

 would involve an intrusion into the outer five miles of the crust 

 of the earth of about 19,000,000 cubic miles of material, and I 

 suspect that this is an underestimate rather than an overestimate 

 of the igneous intrusions in this outer shell of the earth. Suppos- 



