Apeil 10, 1896.] 



SCIENCE. 



569 



lites like the well-known Henry mountains, 

 plutonic plugs of which there are several ex- 

 amples in the vicinity of the Black Hills, and 

 deeply-seated intrusions of a viscous magma 

 which raised vast domes of sedimentary rock 

 with the floor of metamorphic rock on which 

 they rested as the whole Black Hills dome, Big 

 Horn and Park mountains. As to the cause of 

 these uplifts, nothing less than the force exerted 

 hy a cooling globe is thought to be adequate. 

 That they took place very slowly is inferred 

 from the fact that fracture did not result from 

 the bending of thousands of feet of strata. That 

 these domes are in the interior of the continent 

 rather than near the coast is because here the 

 crust is relatively light and strata are hori- 

 zontal, hence pressure on the plastic interior 

 due to contraction of crust or to transfer of ma- 

 terial on the surface would be most likely to 

 produce domes. 



Deformation of Bocks : By C. E. Van Hise. 

 This is the first of a series of papers on the 

 same subject to be published in the Journal as 

 ' Studies for Students. ' The author divides the 

 outer part of the earth into three zones: (1) An 

 upper zone of fracture; (2) a middle zone of 

 fracture and plasticity ; (3) a lower zone of 

 plasticity. Kocks under less weight than their 

 ultimate strength when rapidly deformed are 

 in the zone of fracture. The maximum depth 

 at which fracture can take place is thought to 

 be 10,000 meters. Rocks below this are in the 

 I'egion of plasticity and flowage. Since flowage 

 is necessary to folding, closely folded strata 

 were generally buried beneath other strata. 

 The boundary between the zone of fracture and 

 that of flowage is at different depths for two 

 rocks of different strength, also for the same 

 rock under different conditions of stress, hence 

 there is a zone of combined fracture and flowage. 

 This is thick and of prime importance. In 

 heterogeneous strata in this zone, irregular 

 fracturing, brecciation, jointing, faulting, fold- 

 ing, and development of secondary structures, 

 may occur together in a most complex manner. 

 Between the three zones there are many grada- 

 tions. 



Chas. R. Keyes contributes a careful and 

 appreciative review of Wachsmuth and Spring- 

 er's new book, North American Fossil Crinoidea 



Camerata. Several reviews and authors' ab- 

 stracts of current geological literature follow. 



SOCIETIES AND ACADEMIES. 



GEOLOGICAL CONFERENCE OF HARVARD UNI- 

 VERSITY, MARCH 10, 1896. 



An elementary presentation of the tides: By W. 



M. Davis. 



The object of this communication is to show 

 how the tides may be treated in an essentially 

 scientific manner in an elementari^ collegiate 

 course on physiography. The facts are pre- 

 sented by means of tracings from selected auto- 

 matic records of tide gauges in the Coast Sur- 

 vey office, for stations in mid-ocean (Honolulu), 

 Pacific coast (Port Townsend, Wash.), Atlantic 

 coast (Boston), and in estuaries (Delaware at 

 Philadelphia, and lower Seine, the latter from 

 French records). Mean interval of tides, and 

 systematic variation of interval and of range 

 are numerically determined from these records 

 by the students in laboratory exercises. The 

 agreement of the mean interval with half a 

 lunar day suggests that the moon and the tides 

 may be related in some way as cause and effect. 

 Inquiry is then made as to the manner in which 

 the moon could cause periodic oscillations of 

 the ocean. 



The dimensions, distance and movements of 

 the earth and moon being given, the deforming 

 forces due to lunar attraction, situated as it were 

 on a shell enclosing the earth, are worked out 

 quantitatively in terms of gravity, according to 

 the law of gravitation. A tide opposite to the 

 direct lunar tide, often regarded as an obscure 

 part of the problem, is seen to be as essential a 

 consequence of the theory as the direct tide it- 

 self. The first simple supposition of a moon 

 moving in a circular orbit in the plane of the 

 earth's equator is afterwards changed to the 

 actual condition of the moon moving in an orbit 

 of considerable eccentricity and in a plane ob- 

 lique to the equator ; thus introducing expecta- 

 tions of various systematic inequalities in tidal 

 intervals and ranges. The essential features of 

 diurnal inequality are simply illustrated as a 

 necessary consequence of theory by means of a 

 ' tidal globe,' rigged with appropriate circles 

 for high and low tides. Solar tidal forces and 



