470 



SCIENCE 



[N. S. Vol. XLVII. No. 1219 



country are not necessary consequences of chang- 

 ing climate, but that they, with all their attendant 

 phenomena, are readily accounted for Tvithout 

 recourse to meteoric agencies other than those in 

 active operation in the region at the present time. 

 The genesis of these desert lakes is as varied as 

 that of lakes in the garden spots of earth. 



Some economic mineral deposits of east Tennessee: 

 C. H. Gordon. 



Superficial dip of marine limestone strata: Kirt- 



LET F. Mather. 



Petroleum geologists have long recognized the 

 necessity of distinguishing between the inclination 

 of beds due to purely surfieial causes and that re- 

 sulting from crustal deformation. The latter only 

 is indicative of the underlying structure. Lime- 

 stones may depart from horizontality as a result 

 of groundwater action assisted by gravity. The 

 apparent dip thus caused may extend uninter- 

 ruptedly for considerable distances along hillsides 

 and may closely simulate folded structures. Ex- 

 amples will be cited from the Ordovician lime- 

 stones of Ontario and from the Mississippian lime- 

 stones of Kentucky. The dip of limestone beds, 

 as of all- sedimentary formations, conforms origi- 

 nally to the slope of the floor upon which the bed 

 is deposited. This floor may be quite irregular, as 

 in the case of the pre-Cambrian complex upon which 

 rest the Ordovician limestones of eastern Ontario. 

 Quaquaversal structures in certain limestones near 

 Kingston, Ontario, are due to deposition of those 

 essentially clastic limestones upon the flanks of 

 granite or gneiss hills rather than to tectonic dis- 

 turbances. Submarine erosion contemporaneous 

 with the accumulation of clastic limestones may 

 repeatedly roughen the sea-floor and result in the 

 development of cross bedding on a large scale. As 

 illustrated by Mississippian limestones in Allen 

 county, Kentucky, original dips as great as 12° 

 have thus been caused. Their correct' interpreta- 

 tion may be deduced only when the exposures are 

 unusually extensive and perfect. 



On tlie meclianics of the great overthrusts: Eollin 



T. Chamberlin. 



In the literature of structural geology it is com- 

 monly stated that thrust faulting under compres- 

 sive stress tends to take place along planes which 

 are inclined approximately 45° to the direction of 

 the applied force. With qualifications, this is true 

 of the ordinary reverse fault. But field studies in 

 the last few years have brought to the attention of 

 geologists impressive evidence of the wide preva- 

 lence of a distinctly different type of fault. 



namely the great low-angle overthrust. Its dis- 

 tinguishing characteristics are the very low incli- 

 nation of the fault plane and the extraordinary 

 horizontal displacement often attained. The as- 

 tonishing amount of horizontal displacement is 

 possible because of the low inclination of the plane 

 of slippage which shows no tendency to obey the 

 law of 45° fracture. The low angle of the fault 

 plane seems to afford the key to the problem of the 

 overthrust. An analysis based upon the prin- 

 ciples of mechanics, aided by experimental studies 

 with plaster, paraf^e, clay and sand in various 

 combinations, seems to indicate that the fault 

 plane in the great overthrusts breaks horizontally, 

 instead of at 45°, because of the operation of a 

 number of factors, chief of which are: (1) The 

 normal or direct component of the stress which 

 acts as a frietional resistance to shearing by the 

 tangential component of the stress. With a low- 

 ering of the angle of fracture from 45°, the inten- 

 sity of this frietional resistance is diminished 

 more rapidly than is the intensity of the tangential, 

 or shearing stress. This makes fracturing easier 

 at angles somewhat less than 45°, though the faidt 

 plane remains still far from horizontal. (2) Eo- 

 tational strain, developed from compressive stress 

 (o) in heterogeneous material by bedding, or simi- 

 lar structures, which present just the right differ- 

 ences in competency; (b) in homogeneous mate- 

 rial by (1) any increase in the intensity of the 

 tangential stress in the upper portion of the mass 

 undergoing thrusting with respect to that in the 

 lower portion; (2) by any factors which will 

 lessen the resistance of the surfieial portion while 

 the deeper portion remains less affected, and (3) 

 by any factors which will increase the resistance 

 of the deeper portion subject to thrusting, while 

 the upper portion remains freer to yield. Eota- 

 tional strain is competent to cause fracturing at 

 any angle between 45° and 0°, depending upon the 

 strength of the rotational element. (3) Piling up 

 of material in the first stages of deformation, thus 

 increasing the gravitative or vertically acting 

 force. Acting in conjunction with the horizontal 

 thrusting force, this may cause a lowering of the 

 angle of fracture. (4) Possible minor factors, as 

 heterogeneous material, relatively great length of 

 deformed mass (after analogy of long column), 

 shape, etc. To these factors, operating singly or 

 in various combinations according to the special re- 

 quirements of each particular case, are attributed 

 the peculiarities of the great overthrusts. 



Eollin T. Chamberlin, 

 (To he concluded) Secretary 



