267 



of the falling water are uniform, and strike the bottom with syn- 

 chronous concussion from one end of the dam to the other. It is 

 not surprising that the earth should be felt to vibrate in Spring- 

 field, seven miles in one direction, and in Amherst, fourteen 

 miles in another. 



Another interesting line of inquiry arises here. Hadley Falls, 

 Springfield, and Amherst, are all situated on the red sandstone 

 of the Connecticut valley, I will venture to say that the vibra- 

 tions have not been felt beyond the boundaries of that rock on 

 either side of the valley. Further between Amherst and Hadley 

 we have the intruded trap, forming Mount Holyoke. This trap 

 is said by Prof. Hitchcock not to be a dyke, crossing the strata 

 of sandstone, but to be interposed between the beds of sandstone. 

 This of course is the interjiretation of such parts of the trap as 

 can be examined, but the trap might have cut through the strata 

 of sandstone at some great depth, and only be interposed between 

 the strata near the surface. I believe the fact that these vibra- 

 tions have been felt beyond the trap rock, indicates that it does 

 not cut off" the strata of sandstone at any great depth — that, if it did 

 so, it would cut off the vibrations. Now if the trap lies entirely be- 

 tween strata of sandstone, it may not have been injected, but it 

 may have been poured over the surface of the underlying stratum 

 when that was the uppermost. 



Dr. C. T. Jackson, instancing the vibrations noticed at 

 the Dam at Nashua, stated, that, in that place, the vibra- 

 tions only take place when the wind is in such a direc- 

 tion as to break the fall, and permit of the escape of air, 

 which is evidently confined behind the sheet of water. 



Mr. Theodore Lyman read an extract from a letter of 

 Mr. Bowerbank of London, acknowledging the reception 

 of some sponges sent him by the Society, and promising 

 to send a collection of sponges in return. Mr. B. is de- 

 sirous of obtaining specimens of a spongilla which he 

 says he learns is common in the Cochituate water-pipes 

 of Boston. 



