Mat 15, 1914] 



SCIENCE 



707 



which there have been great depositions are 

 in isostatie equilibrium. We, therefore, 

 must conclude that the mountains have 

 been maintained for long ages at approxi- 

 mately the same elevation by an undertow 

 of material toward them, while the areas of 

 deposition maintain their depth in spite of 

 the added material by a sinking due to the 

 undertow from the crust under such areas. 

 Consequently, the continent may be consid- 

 ered as not being leveled off as a result of 

 erosion, but as kept at approximately the 

 same general elevation. 



While an isostatie adjustment by means 

 of a transposition of material may account 

 for the normal condition under the areas 

 of erosion and deposition, it does not ac- 

 count for the great elevation, into moun- 

 tains and plateaus, of rock which was once 

 under the ocean. Evidently there must 

 have been a change in the density, under 

 such areas, and an expansion of the mate- 

 rials in the earth's crust to the extent indi- 

 cated by the height of the topography 

 above the sea. This change was not due to 

 a mere expansion from change of tempera- 

 ture, as a bar would expand. The expan- 

 sion of the crust must have been due to 

 chemical or physical changes which made 

 the elements present form compounds with 

 densities differing from what they had been 

 previously. What could cause such rear- 

 rangement as to change the density by sev- 

 eral parts in one hundred is a problem for 

 the geophysicist and geologist. 



The fact that material which was once 

 under the sea now forms a mountain 

 range seems to disprove the "root of the 

 mountain" theory of compensation. It is 

 difficult or impossible to see how the crust 

 with its surface at or below sea level (sup- 

 posedly in isostatie equilibrium) could, in 

 addition to expanding to form the moun- 

 tains, also decrease its density in that por- 

 tion far below the average depth of com- 



pensation (122 kilometers). As a matter 

 of fact, the deflections and the gravity 

 observations indicate that the compensa- 

 tion is not deep down in the crust, but 

 in general within the depth stated above. 



The movements noted in the exposed 

 earth's materials have been slow and were 

 in most cases due to stress differences 

 which were below the rupturing point of 

 the rock. There has been much slow yield- 

 ing, without fracture, to long-continued 

 stress differences. The isostatie adjustment 

 of the earth's materials in the upper crust 

 after or during erosion might cause many 

 of the movements that have been recorded 

 in the strata. 



In conclusion, the immediate work which 

 the geodesists should perform is to reduce 

 all the existing gravity stations of the 

 world by the same method which should 

 be based on a rational theory of isostasy. 

 As opportunity permits, a gravimetric 

 survey should be extended to new regions 

 and especially to oceanic islands. And as 

 soon as an apparatus is designed which may 

 be used successfully on a vessel, an exten- 

 sive gravimetric survey of the oceans should 

 follow. 



William Bowie 



U. S. Coast and Geodetic Survey 



SAFEGUARDING THE HEALTS OF COLLEGE 

 STUDENTS^ 



Attention has frequently been called to the 

 fact that many educational institutions fail 

 to make adequate provision for safe^arding 

 the health of their students. These criticisms 

 find expression most often in connection with 

 the report of the injury or death of a student 

 who has been allowed to participate in ath- 

 letics without proper medical supervision; or 

 when some serious sanitary fault causes, or 

 contributes to, an epidemic among the stu- 



1 Based upon paper presented at Fourth Inter- 

 national Congress on School of Hygiene, Buffalo, 

 1913. 



