GEOLOGY. 289 



by the frost or other expansive force. The same caution should be 

 observed with micaceous or gneissoidal granites. 



The general conclusion arrived at is, that the first two specimens 

 possess nearly the same character of cohesion and hardness, but the 

 less susceptibility of the New Brunswick specimen io absorb water is 

 certainly in its favor, more than counterbalancing the injurious effect 

 of the iron. The absence of mica is also in its favor, while the ab- 

 sence of carbonate of lime in both give them a decided superiority 

 over the foreign article. Trinity Church in New York, and the Athe- 

 naeum in Boston, are built of the New Jersey stone. The Connecti- 

 cut variety is probably of less value than either of the others. 



Mr. Alger directs attention to the experiment recommended by Dr. 

 Ure, for testing the durability of sandstone, which, it is said, has 

 proved very successful in Great Britain, though its application in this 

 country may not be followed by the same results, owing to the greater 

 severity of our climate. It consists in immersing a portion of the 

 stone in a saturated solution of sulphate of soda, and if then exposed 

 to the air for some days, crystallization will take place within the 

 stone, and cause the same disintegration that would follow from the 

 influence of frost. 



DURABILITY OF STONE. 



PROF. WALTER R. JOHNSON of Washington, D. C., communicates 

 to Sillimari's Journal for Jan., 1851, an article on the comparative 

 strength and durability of various American and foreign building 

 stones, with a special reference to the durability and nature of the 

 stone used in the construction of the Washington Monument, at 

 Washington. At a meeting of the American Association, in 1849, 

 Prof. Johnson presented a communication in regard to the unfitness of 

 the material used in the construction of the monument referred to.* 

 The facts then stated by Prof. Johnson having been repeatedly called 

 in question, the following abstract of the article in Silliman's Jour" 

 nal will, we think, have a tendency to settle the disputed points. 

 Editors. 



Those rocks which, amid decomposing influences, whether derived 

 from currents of water, meteoric agencies, or vegetable growth and 

 decay, have been able to sustain themselves in high, naked, and angu- 

 lar cliffs, unprotected by soil, and yet unfurrowed by irregular disinte- 

 gration, are manifestly those to which the engineer and architect are 

 to direct their attention, when they seek materials for durable works 

 of art. On the other hand, they will shun those rocks which the 

 causes above enumerated have kept constantly down to a level with 

 the ground, or which barely rise in some few patches to the surface, 

 and are then seen disintegrating, scaling away, and covering them- 

 selves with a soil derived from their own debris. The stone used in 

 the construction of the Washington Monument is generally known as 

 the alum limestone, and is quarried about 13 miles from Baltimore. 



* See Annual of Scientific Discovery, 1850, p. 275. 

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