THE GEOLOGICAL EVIDENCES. 53 



Essentially all of the other assignable causes of deformation of the 

 major class seem to be general in their application and to affect all latitudes 

 practically alike. This is true of cooling, of internal redistribution of heat, 

 of molecular rearrangement, whether chemical, crystalline, or diffusive, of 

 atomic transformations and decompositions, of radioactivity and presum- 

 ably of igneous extravasations. Grant to these agencies whatever sep- 

 arate or combined effect may be their due, that effect, if it be general 

 and essentially indifferent to latitude, as it seems that it must be, should 

 be distinguishable from the effect of a superimposed agency that is pro- 

 nouncedly correlated with latitude, because of this peculiarity. Granted 

 a given amount of uniform earth shrinkage as the result of the general 

 agencies named, or any of them, the crustal stress arising from this in the 

 equatorial belt would be intensified by the addition of the stress of the same 

 kind arising from the retardation of the earth's rotation, while the crustal 

 stress which arises from these agencies in the polar regions would be propor- 

 tionately relieved by the tension arising there from rotational retardation. 

 A difference of result equal to the algebraic sum of the retardational and 

 general stresses should be manifest in the resulting deformations. The 

 conspicuousness of this difference must depend largely on the relative values 

 of the two classes of agencies, which is our second point of consideration. 



If, on the tidal side, we take the higher deformative values given in 

 Slichter's table (page 67), and if, on the other side, we take estimates of 

 shrinkage made from a study of the foldings and faults of the earth, a 

 comparison may be made. Quite without thought of this application, I have 

 recently reviewed the data of the latter class in the endeavor to form a 

 reasonable estimate of the amount of shrinking which the earth has prob- 

 ably undergone; and, while this estimate has little claim to value in itself, 

 it may perhaps be taken to fairly represent the import of the present 

 imperfect data. It is as follows: 



If one is disposed to minimize the amount of folding, the estimate may perhaps be 

 put roundly at 50 miles, on an entire circumference, for each of the great mountain-making 

 periods. If, on the other hand, one is disposed to give the estimates a generous figure so 

 as to put explanations to the severest test, he may perhaps fairly place the shortening at 

 100 miles, or even more. For the whole shortening since Cambrian times, perhaps twice 

 these amounts might suffice, for while there have been several mountain-making periods, 

 only three are perhaps entitled to be put in the first order, that at the close of the Paleo- 

 zoic, that at the close of the Mesozoic, and that in the late Cenozoic. The shortening in 

 the Proterozoic period was considerable, but is imperfectly known. The Archean rocks 

 suffered great compression in their own times, and probably shared in that of all later 

 periods, and if their shortening could be estimated closely, it might be taken as covering 

 the whole. Assimiing the circumferential shortening to have been 50 miles during a given 

 great mountain-folding period, the appropriate radial shrinkage is 8 miles. For the more 

 generous estimate of 100 miles, it is 16 miles. If these estimates be doubled for the whole 

 of the Paleozoic and later eras, the radial shortening becomes 16 and 32 miles, respectively.* 



If we assign to the Proterozoic era a shrinkage equal to the Paleozoic, 

 Mesozoic, and Cenozoic eras combined, and to the known Archean twice as 

 much, the minimum and maximum estimates are 64 miles and 128 miles 

 of radial shrinkage, respectively, or roundly 400 and 800 miles circumfer- 

 ential shortening, respectively. 



* Geology, vol. 1, Chamberlin and Salisbury, 1904, p. 551. 



