June 12, 1908] 



SCIENCE 



925 



probably filled by material moved out from the 

 adjacent synclines so that the structure from 

 one syneline to another actually approached 

 a convex arch between two concave ares. The 

 total structural shortening northwest of the 

 Allegheny front was between 35 and 40 feet. 



The faulting in the Allegheny tract from 

 Alabama to Pennsylvania shows remarkable 

 parallelism in lines approximately 45° from 

 the strike of the folded structure or from the 

 direction of the force causing the folding. 

 These faults were always of the type known as 

 " normal " ; but the fault faces showed striae 

 running from 3Y° to 65° from the horizontal, 

 indicating a large horizontal element in the 

 motion. Although entirely normal, the dis- 

 locations were evidently due to the yielding 

 of the rocks under high tangential pressure 

 along stress planes. 



Secondary Enrichment in Granite-Bimetallic 

 mine, Philip shur g , Montana: W. H. Em- 

 mons. 



The Granite-Bimetallic lode is a tabular 

 body of silver ore from 1 to 20 feet wide 

 which has been stoped for 4,500 feet along the 

 strike and 2,600 feet in depth. The vein fills 

 a fissure in monzonite along which there has 

 been but little movement. The primary ore 

 has a gangue of quartz and rhodoehrosite in- 

 closing a large quantity of pyrite, arsenopy- 

 rite, tetrahedrite, and termantite, with some 

 galena and zinc blende. Sparingly scattered 

 through this ore are small specks of pyrargy- 

 rite, realgar and orpiment. This ore carries 

 from 20 to 30 ounces of silver and from $1.50 

 to $3 in gold. Above the low grade sulphides 

 extending from 500 to 1,000 feet below the 

 surface is a zone of enriched oxide and sul- 

 phide ore in which the primary ore is cut by 

 veinlets of ruby silver, argentite, native 

 silver and horn silver. This ore carries over 

 100 ounces silver and $5 gold. Above the rich 

 oxide and sulphide zone is a zone of leached 

 oxides which extends to the surface. This 

 ore carries less than 40 ounces silver and $2 

 gold. 



An analysis of the mine water from a long 

 drainage adit shows that it contains .000,147 

 SO,, .000,160 iron with traces of silver and 



gold. These figures indicate that about .6 of 

 a ton of iron and sulphate compounds are 

 carried out of the mine every twenty-four 

 hours. It is well known that iron sulphate 

 solutions will dissolve silver and that these are 

 precipitated again by the action of sulphides 

 which carry an excess of sulphur. Enrich- 

 ment was favored by a relief which furnished 

 an ample head; by fracturing of the vein 

 subsequent to the deposition of the primary 

 ore; and by abundance of sulphides in the 

 primary ore. 



Revision of the Age of Niagara Falls: Mr. J. 



W. Spencer. 



From the now partially buried remnants of 

 the river banks Mr. James Wilson, C.E., and 

 the writer were able to locate the position of 

 the falls at the time of Father Hennepin's 

 visit (1678). It was thus found that the rate 

 of recession in 227 years had been substan- 

 tially the same as during the 63 years between 

 the survey of Hall (1842) and of the writer 

 (1904^5), namely, an average rate of 4.2 feet 

 a year. The recession of the American Falls 

 is very slow, and this water added to the main 

 falls would increase them by only five per 

 cent. 



Through soundings it was found that the 

 present mean rate of recession of 4.2 feet a 

 year has obtained only since the Falls passed 

 a point 1,100 feet below the apex, with an 

 effective height in recession taken at 180 feet. 

 The time required was 260 years. Thence, 

 northward, for 6,200 feet, the mean effective 

 height (on account of the absence of the 

 barrier at the Whirlpool Eapids) was 240 feet, 

 with the rate of recession thereby increased to 

 5.6 feet a year. Beyond, for 10,200 feet, to 

 near Sinclair Point, the effective height was 

 260-280 feet, increasing the rate of retreat to 

 6.5 feet a year, with the necessary time ratio 

 of 1,570 years. As the Whirlpool only re- 

 quired the clearing away of the drift material, 

 no time allowance is made for its opening. 

 From its outlet to the head of Foster Flats 

 (3,200 feet), with the water descending 240 

 feet, the rate was 5.6 feet a year, requiring 

 570 years. Thus, the Falls have receded 

 about four miles, with the full volume of the 



