Apbil 2, 1909] 



SCIENCE 



555 



posed to deny that this may have happened. 

 But I eonsider it much more probahle — as 

 does Mr. Gordon apparently— ;-that most, if 

 not all, of our otters (at least those found in 

 eastern Massachusetts) are descended from 

 primitive native stock. For it is evident that 

 the species* has never been completely extir- 

 pated in Massachusetts, even in the neighbor- 

 hood of such large cities as Boston and 

 Springfield, while any assumption that there 

 have been immigrations from farther north is 

 unsupported by known evidence and also un- 

 satisfactory because of the fact (to which I 

 could bear strong testimony if it were neces- 

 sary) that in most parts of northern New 

 England otters are, and have been for twenty 

 years or more, far from common. Hence it is 

 difficult to believe that many of them have 

 come to us from that direction, although a 

 very few may stray southward, at infrequent 

 intervals, along the Connecticut and Merrimac 

 rivers. However this may be, I am decidedly 

 of the opinion that if, within recent times, 

 there has been anything in the nature of an 

 overflow of otters from localities which they 

 have somewhat over-populated, its source is 

 most likely to have been Cape Cod. For there, 

 as I have said, the otter has been more numer- 

 ously represented, over wide areas, during the 

 past quarter of a century, than anywhere else 

 in New England. 



William Brewster 

 145 Beattle Steeet, 

 CAMBEnjGE, Mass. 



SOCIETIES AND ACADEMIES 



THE GEOLOGICAL SOCIETY OP WASHINGTON 



At the 212tli meeting of the society, held on 

 Wednesday, January 13, Mr. G. K. Gilbert pre- 

 sented the following paper : " Earthquake Fore- 

 casts," a paper read before the Association of 

 American Geographers at Baltimore, on Friday, 

 January 1. This paper was published in Science 

 and hence no abstract is furnished herewith. 



At the 213th meeting of the society, held on 

 Wednesday, January 27, the following papers 

 were presented: 



Regular Program 

 Some Oiservations on Rocky Mountain Faults: 



Chestee W. Washbuene. 



The faults observed may be referred to three 

 genetic types. 



Type I. Normal dip faults crossing the aa>es 

 of anticlines. — The examples considered displace 

 strata just above the Colorado shale, and do not 

 penetrate the latter over 300 feet. None reach 

 the Carboniferous. The maximum vertical dis- 

 placement, 50 to 300 feet, is on the anticlinal 

 axes and decreases down the limbs until the 

 faults become small monoelinal flexures and 

 finally disappear. The motion along each fault 

 plane had two essential components : ( 1 ) vertical, 

 either (a) upward movement of the foot- wall, or 

 (6) downward movement of the hanging wall; 

 and (2) horizontal, either (a) inward movement 

 of the foot-wall toward the anticlinal axis, or 

 (6) outward movement of the hanging wall away 

 from the axis. Field observations show that these 

 movements have been combined in one of two 

 ways. A: (la) with (2a); or B: (15) with 

 (26). Combination A would be produced during 

 folding by compression resulting in axial thicken- 

 ing of the underlying shale and upward creep 

 on the limbs, the movement being greater on the 

 up-thrown or foot-wall side of each fault. Com- 

 bination B would be produced by creep down the 

 limbs of anticlines and axial thinning of the Colo- 

 rado shale, the movement being greater on the 

 down-thrown side. This might be caused during 

 the folding, by the tension of stretching over the 

 underlying Paleozoic limestones, or subsequently 

 by gravitative creep down the limbs, under the 

 pressure of about 3,000 feet of overlying rock. 

 Combination A is the most probable. The breaks 

 were probably initiated as lines of scission or 

 blatter between blocks that were shoved unequally. 



Type II. may, for convenience, be called faults 

 of vertical thrust, because the vertical component 

 is large. The type is characteristic of the margins 

 of broad, domical, flat-topped uplifts such as the 

 Marysville batholith and the Bighorn Mountains. 

 The faults are usually intimately associated with 

 flexures. The faults that are of the same kind, 

 orographically, as the flexures, i. e., the faults 

 that add to the height of the uplift and to the 

 depth of the adjacent depression, are named 

 additive. These are due to the same forces that 

 elevated the mountain mass. The additive group 

 includes both " thrust " and " normal " faults of 

 the current nomenclature. The complementary 

 group of faults subtracts from the height of the 

 uplift and is therefore designated subtractive. 

 They are due to forces opposite in kind to those 

 that made the uplift, probably to gravitative 

 subsidence. All subtractive faults are " normal." 



