August 18, 1922] 



SCIENCE 



181 



some facts regarding the structure of rocks 

 whicli lie more exclusively in the province of 

 geology. The first of these is what is called 

 "jointing" in rocks. Jointing is a mechanical 

 effect. It is produced by pressure in the case 

 of massive rocks, like granite, or by torsion in 

 the ease of strata, when they are warped. All 

 the rocks of the plateau region are jointed. In 

 the magnificent architecture of the Grand Can- 

 yon, the vertical cliffs are the planes of joints. 

 In the pointed forms which are characteristic 

 of the deepest gorges in the granite, we see the 

 effect of two or more intersecting joint planes. 

 Now joints permit water to penetrate under 

 and around a block of rock. The film of water 

 may be very thin, but to the extent that it sur- 

 rounds the rock it buoys it up, tends to lift it 

 from its bed by virtue of hydrostatic pressure, 

 and may free it from its firm foundations. 

 Engineers are fully aware of this action. They 

 seek to excavate to foundation rocks which 

 show no open joints, or to seal visible joints 

 by cement. Granite is regarded as one of the 

 firmest foundations. It is liable, however, to 

 blind joints, invisible planes on which there 

 has been no actual parting, but the minerals 

 have been strained and are ready to react to 

 forces of decay. Water, not enough to wet, but 

 just enough to moisten, is the agent that sets 

 those forces to work. The engineer can not 

 discover blind joints. Investigation of the 

 minerals by the microscope, a study which is 

 among the most specialized of geologic train- 

 ing, alone can demonstrate whether or not they 

 are present. 



We may think that the invisible is reason- 

 ably negligible. But the infinitely small is the 

 infinitely powerful and also the infinitely pa- 

 tient. A film of water penetrating a plane of 

 strained crystals may open the way to the ulti- 

 mate destruction of man's mightiest work. 



Minerals decay. That is not a familiar 

 thought with many, although soil, a product of 

 mineral decay, is familiar to every one. I said 

 that minerals are alive. And it is because they 

 are alive that they decay, decay being simply 

 the reaction to a change of environment. Evi- 

 dence of these facts is found in the rocks which 

 the engineer must use in building a dam in 

 ;:.v. Ciiiivon ci the CjIo:-ado. Tie. ^'ani'e" and 



schists of the ancient formations crystallized 

 deep within the crust in an environment of 

 very high temperatui'e and great pressure. 

 Elevated to their present positions at the sur- 

 face they are in a cool environment, under 

 little pressure. The change produced in every 

 crystal a tendency to change, to disintegrate 

 into forms and compounds better suited to 

 their actual environment. Thus the seemingly 

 solid granite of the Boulder Canyon site is 

 pervaded with disintegrating forces, which 

 will in the course of time, though probably 

 long time, certainly cause it to crumble. 



Let us now consider the two methods of 

 building the proposed dam, which have been 

 suggested. The first fascinates by its unusual 

 character and its daring. It consists in blow- 

 ing great masses of rock from the canyon walls 

 into the river channel in such quantity that 

 they will form the body of a dam three quar- 

 ters of a mile up and down stream on the base 

 and six hundred feet high. The blasting is not 

 to be done all at once, but in sections from 

 upstream downward and only to a part of the 

 height at any one time. And the river is to be 

 allowed to flow over the fallen rock masses in 

 such manner that it will by its own power dig 

 the deep hollows into which it shall also roll 

 the great rocks. Thus the river shall work its 

 will, but shall lay the masses where it can 

 never move them again. It shall chain itself. 



The success of this operation depends upon 

 providing by blasting rock masses of such 

 magnitude that the river can not carry them 

 away. We have a report by Mr. Ransome, a 

 geologist of the U. S. Geological Survey and 

 one of the highest rank, to the effect that the 

 granite of Boulder Canyon is considerably 

 jointed. Conservative knowledge would sug- 

 gest that there are many more joints than ap- 

 pear as actual fissures and that they would 

 cause the granite to break into relatively small 

 masses, in the blasting from the cliffs and sub- 

 sequent rolling by the river. 



Another question is how "solid" is the 

 granite? That it is so described is natural, 

 for "granite" and "solid" are almost synony- 

 mous terms to English speaking peoples. The 

 significance of words depends upon our asso- 

 '•i.ition with tliem and our experience of New 



