404 



SCIENCE. 



[N. S. Vol. XXIV. No. 613. 



THE INTERIOR OF THE EARTH. 



To THE Editor of Science: Professor 

 Thomson's recent article on the cause for 

 volcanic action, which begins with a reference 

 to the theory of a liquid interior of the earth 

 as exploded, taken in connection with a recent 

 paper by my friend. Dr. Daly, has set me 

 thinking as to what we mean by speaking of 

 the interior of the earth as either solid or 

 fluid. That if fluid it is in general highly 

 viscous is certain; that if solid it is also often 

 subject to stresses above its crushing strength, 

 under which it flows like punched steel, all 

 will agree. But are we clear as to where we 

 are to draw the line between viscous fluid and 

 solid? We have, on the one hand, substances, 

 which, like the splinters of albite described 

 by Day, are so viscous as to retain their form 

 even when heated so that they have lost their 

 crystalline structure and then become optic- 

 ally homogeneous, and we have, on the other 

 hand, experiments like those of Adams on the 

 flow of rock solids. I do not think we can 

 come to any clear idea or definition without 

 emphasizing the relation of time. Molasses 

 candy and tar appear ordinarily as solids and 

 crack and break as such, yet given time 

 enough they flow, and are properly classed as 

 viscous fluids. In geologic problems we have 

 large quantities and long times at disposal. 

 The argument for the solidity of the earth 

 from its resistance to the pull of the sun and 

 moon proves that it is highly rigid, but not 

 at all that it may not be a viscous fluid, be- 

 cause the time in which the stresses are ap- 

 plied is relatively brief. 



The scientific distinction between solid and 

 fluid may be derived from the common idea of 

 a solid as that which has a form of its own, 

 while the fluid takes the form of the contain- 

 ing vessel, bounded by a level surface on top. 

 It is obvious that we must give it time to take 

 the form in question. A very viscous fluid 

 may take a very great time, yet it will ulti- 

 mately assume the same form as the other 

 fluid. Whence we may base an ideal defini- 

 tion and say that anything to be classed as 

 fluid will, in sufiicient time, come to the same 

 state of equilibrium as water, and is not able 



to rest in a state of strain, hut yields to a 

 stress hoivever small or slowly applied. 

 Whereas a solid, such as a crystal of quartz, 

 might be under a light stress producing a 

 slight strain for infinite time without any 

 tendency to permanent distortion. 



For geological purposes we do not need to 

 deal with quite infinite time. While stresses 

 such as those of the tides and volcanic earth- 

 quake shocks are far too quickly applied to 

 discriminate between viscous fluids and solids, 

 the slow erosion of a. continent by the fraction 

 of a thousandth of a foot a year, the slow 

 attendant deposition of sediment on the ocean 

 floor and the slow escape of energy from the 

 interior of the earth are examples of forces so 

 slowly applied that if there is any degree of 

 real fluidity worth mentioning in any layer 

 of the earth, there could be no accumulation 

 of such stresses in it, but they would be yielded 

 to as fast as formed. 



According to a geological theory known 

 as that of isostasy, there is something like 

 this continuous yielding in the case of erosion 

 and deposition, the continents being uplifted 

 as they are eroded, while the ocean shores are 

 depressed, so that continents float above the 

 general earth level because they are lighter, 

 like icebergs in water, and not because they 

 are supported by stresses in the earth beneath. 

 In the same way the earth is supposed to have 

 been depressed beneath the icecap and to have 

 risen again, tilting the shore lines of the 

 Great Lakes, when the ice melted away. If 

 this theory is strictly true it would seem to 

 me that we must assume a viscous fluid sub- 

 stratum. But is this process absolutely con- 

 tinuous, or only ' steady by jerks,' the yielding 

 occurring only after a certain degree of strain 

 is accumulated? Upon the possibility of 

 accumulation to some extent of very slow 

 strain should depend the answer as to whether 

 there is a viscous fluid or plastic solid sub- 

 stratum of the earth. As President McNair 

 has suggested to me, there is an experiment 

 now being unwillingly tried on a gigantic 

 scale which might throw light on this. The 

 Colorado Eiver is about to cover some 2,000 

 square miles in the next 30 or 40 years with 



