210 T. Mellard Reade — A Cooling and ShrinJcing Globe, 



On the coDtrary, we find that the cooling shell of the globe for a 

 depth of 145 noiles below the level-of-no-strain is actually con- 

 tracting, but in degrees, varying with vertical position ; that the 

 stresses are tensile, rupture being avoided by the gravitation of 

 the overlying masses closing up otherwise possible cavities by com- 

 pressive extension. 



But if we look at the globe as a mass which is not losing heat 

 equably but is contracting at one locality and expanding at another, 

 though still on the whole undergoing secular refrigeration, it will 

 not help what is known as the "contraction theory" of mountain 

 building. The effects calculated in this paper as possibly due to the 

 condensation of the shell of compression will be less, more irregular, 

 and difficult to trace out. Indeed, so much so that the disturbing 

 causes will have far greater effect than general secular contraction, 

 and while adding to the lateral movement in one place may efface it 

 in another. 



Effects of unsymmetrical cooling on the outer envelope of tJie Earth. 



Having now, as far as my limited powers of exposition allow, 

 stated for your considei-ation my views on the effects of symmetrical 

 cooling on a homogeneous heated globe, comparable in size with our 

 earth, and partly traced some of its possible geological effects on the 

 Outer envelope, it is time to consider quantitatively some of the dis- 

 turbing agencies which are constantly at work to modify the sum of 

 the results. 



First then, and most important as initiating other movements, is 

 that of sedimentation. Those who are not familiar with geology will 

 pardon me for stating that the gross thickness of all the sedimentary 

 formations of the earth is estimated by competent authorities at as 

 much as 1<X),000 feet/ but it must be understood that these never 

 occur together in one geological column. The statement is sufficient 

 to invest sediments with considerable interest to physicists as modi- 

 fying in several important ways reasoning based on hypothetical 

 conditions, though they are hypotheses that are absolutely necessary 

 to be made before the actual problem can be efficiently attacked. 



The greatest actual thickness of sediment piled up in one spot in 

 almost unconformable succession is found in great mountain ranges 

 such as the Appalachians and the Alps, and has been estimated by 

 competent geologists at from 8 to 10 miles.^ 



But below these is a mass of Archean rocks of unknown thickness 

 which is certainly not the original unmodified crust of the globe. 



The area of the sedimentaries is co-extensive with the continents 



^ Sir A. Geikie, Pres. Address British Association, 1892. 



^ Jiidd, speaking of the Alps (Volcanoes, p. 295), says : " The united thickness of 

 all sediments aocumulated along this great line of subsidence between the Permian 

 and Nummulithie periods probably exceeds 50,000 feet or 10 miles." Various 

 American authorities estimate that the thickness of the Palaeozoic system of the 

 Appalachians reaches 40,000 feet ; the Palaeozoics and Mesozoics in the "Wasatch 

 50,000 ; the Cretaceous in the coast ranges of California 20,000 feet, and in Shasta 

 county 30,000 feet; the Palaeozoic and Mesozoic of the Uinta 30,000. — See Le 

 Conte — Theories of the Origin of Mountain Ranges, Journal of Geology, 1893, p. 544. 



