240 RECORD or SCIENCE FOR 1887 AND 1888. 



During the biennial period there have been some valuable foreign 

 contributions to our liiiowledge of the causes and conditions of niount- 

 aiu-uuiking, notably the development of the conception of the " level- 

 of-no-strain;" for as shown upon i)hysical grounds, by Eeade, Fisher, 

 G. H. Darwin, and Davison, there is at limited depth within the terres- 

 trial crust a horizon or couchc in which tangential stress disappears. 

 This conception has modified American as well as foreign thought, but 

 thus far no important contributions to the subject have beeu made on 

 this side of the Atlantic. 



Apropos to the conclusions of geologists and physicists concerning the 

 isostatic condition (or condition of hydrostatic equilibrium) of the exte- 

 rior crust of the earth, ami of the bearing of these conclusions upon the 

 general problem of mountain-making, reference may be made to a prac- 

 tically new conception as to certain relations between sedimentation 

 and depression which bears upon the theory of mountain-making re- 

 cently advocated by Reade. It may be thus stated : Lines of sedi- 

 mentation are the margins of continents, and the sediments are laid 

 down not upon horizontal surfaces, but upon seawardly sloi)ing bot- 

 toms ; so the sediments do not form horizontal beds, but take a variable 

 seaward slope, determined by marine currents, wave action, etc. Thus 

 the mass of sediments is collectively in the condition of a mass of snow 

 upon a roof or upon a mountain side ; i. e., in a condition of potential 

 instability or inequipotentiality. If the mass is stable in either case, it 

 is because the friction among the particles exceeds the attraction of 

 gravitation upon the particles; it is obvious that if i)article friction 

 were reduced by augmentation of temperature or by alteration of con- 

 stitution, or if the efticieucy of gravitation were increased by addition 

 to the mass, tlie point of stability might be passed, when the mass 

 would move in the direction of the slope; and it is equally obvious 

 that if an inequipotential mass expand, the resulting movement will 

 not take place equally in all directions, but mainly or wholly in the 

 direction of least resistance, which is that of the slope. Since the 

 sediments fringing continents are in a condition of inequipotentiality, 

 any movement due to the rise of isogeotherms or other cause must take 

 place in a single direction ; and it might not be limited to that due to 

 expansion, for other factors co-operate. Supplemented by this addi- 

 tional conception, the hypothesis of mountain growth so ably advocated 

 by HerscheP (who alone recognized vaguely the conception), Eabbage, 

 Hall, Dana, Le Conte, Reade, and a score of others, appears to gain 

 much in acceptability.t The great displacement of the Middle Atlan- 

 tic slope has been attributed to downward and seaward settling of the 

 inequipotential mass of Mesozoic and Cenozoic sediments constituting 

 the coastal plain.^ 



* L. E. & D. Phil. Mag., 1856, 4th ser., xii, pp. 197-198. 

 t McGee, Geol. Mag., Decade III, 1888, vol. V, pp. 494,495. 

 : 7th Auu. Report U. S. Geol. Survey, 1888, p. 634. 



