Niles.] O0 [January 4, 



tliose upon the edge of the slab were one-fourth of an inch further to 

 the north, than those of the parent rock. 



Prof. Johnston, of Middletown, Ct., has described 1 " some sponta- 

 neous movements occasionally observed in the sandstone strata in one of 

 the quarries at Portland, Ct." The movements there observed, he 

 says, are the sliding of one stratum upon another. These occur 

 whenever a stratum at the bottom of the quarry is first broken by 

 excavating a channel in it, which trends in easterly and westerly di- 

 rections. When such a channel has been cut only partly through 

 the thickness of the stratum, the stone remaining at the bottom is 

 crushed with a loud report, and the edge of the northern side moves 

 southward about three-fourths of an inch. " These facts," he thinks, 

 " plainly show that the strata of sandstone at this place are not, at 

 the present time, perfectly at ease in their ancient bed," but he does 

 not attempt to determine the cause. 



I am not aware that any other scientific man has observed and 

 recorded facts similar to those given by Prof. Johnston. Probably 

 such phenomena are to be observed at comparatively few localities. 

 At many quarries the rock is so divided by loose joints, that it would 

 quietly yield to a force, which at Monson would cause the phenomena 

 described. From some statements made to me by persons interested 

 in quarrying extensive and comparatively unbroken strata, I am 

 inclined to believe that facts of a similar character might be 

 collected. ► 



When I visited Portland, the condition of the quarries gave me no 

 opportunity for observing phenomena of the kind described by Prof. 

 Johnston. T found, however, that the facts were well known to 

 those who had been engaged there for a long time, through whom 

 I was able to determine that the movements were from east of north 

 towards west of south, or very nearly parallel to the directions of the 

 movements at Monson. The force causing the phenomena at the two 

 localities may be identical, for the manner of working the quarries at 

 Portland gives no opportunity for observing fractures, elevations and 

 expansions, like those so well shown at Monson. 



Certain features in the structure of rocks, as cleavage, for example, 

 have been considered, by eminent geologists, as evidences of severe 

 compression ; also, polished and striated surfaces have been regarded 



1 Proc. Am. Association for the Advancement of Science. Eighth Meeting. 

 1854. p. 283. 

 National Magazine. Vol. in, No. i. Oct., 1853. p. 362. 



