AMERICAN ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE. 75 



as "Were in the line, many of them being of such size as to require blasting to break 

 them up, and some being actually removed to the required distance from the line 

 by heavy blasts. 



The base apparatus already described before the association and described and 

 figured in my report for 1854, by Lieut. E. B. Hunt, of the Corps of Engineers, 

 was used in this measurement. 



The measurement was begun at the west end of the line on Saturday, the 18th 

 of July, but the next week proved so rainy that it was only resumed in earnest on 

 Monday, 2'7th. 



The work of the first Saturday (24 tubes) was measured on the following Mon- 

 day with precisely the same result as to length, the end measurement falling 

 precisely on the marks which had been placed as terminating the first. The mark 

 was placed and verified as all others of the same sort in one measurement by a 

 transit placed at right angles to the line and at a moderate distance from it. 



This was a descending slope of the strongest grade adopted, and there was a 

 difference of temperature of some five degrees in the two measurements. Oil 

 Tuesday a length of 18 tubes which had been measured on Monday was re-measured 

 with an identical result. This was on an ascending slope. 



On Monday the work was in part interrupted by the arrangements for photo- 

 graphing the apparatus, on Tuesday by a fog, and on Wednesday by showers in the 

 morning ; we made, however, half a mile each day. 



On Wednesday began a series of four unbroken days, during the first of which 

 we measured -f of a mile, and on the three others a mile or more than a mile each 

 day, reaching the east end of the Base on Monday evening. 

 Whole length of Base 28,601 feet, or about 5.4 miles. 

 Mean level of Base above mean tide 25Y feet. 



Approximate correction for reduction to the level of the sea 4 inches nearly. 

 No. of tubes inclined 647 



" " level 810 . 



Correction for versed sine for whole base, 9.2 feet to be subtracted. 

 Maximum inclination 3^14' 



Greatest day's work 281 tubes, 1.05 miles, in llh. 10m. working time, averaging 

 1 tube in 2m. 278. 



DEPOSITION OF NATIVE METALS IN VEIN FISSURES, &0., BY ELECTRO-CHEMICAL AGENCY. 

 BY PROF. E, J. CHAPMAN, OF UNIVERSITY COLLEGE, TORONTO. 



From the known fact that solutions of various metallic salts may be decomposed 

 by voltaic agency, and the metal obtained in the simple state, it has long been a 

 favorite theory with many geologists, that depositions of native metals, in veins, 

 &c., are due to a similar cause. That such may be a perfectly legitimate conclu- 

 sion in many instances, I am quite ready to admit ; but, in applying this view to 

 any particular case, it is necessary, unless the explanation is to be regarded as a 

 mere theory of convenience, that certain collateral circumstances be not altogether 

 excluded from consideration. If these circumstances oppose themselves to our 

 theory, and remain by it altogether unanswered ; nay, if but a single well-proved 

 fact withold its concurrence from the conditions demanded — surely it is more con- 

 sistent with our obligations to scientific truth, that we abandon the theory at once 

 —however plausible in itself, and however convenient in its application — rather 



