1873.] 41 [Niles. 



Dr. T. M. Brewer presented by title the following paper, 

 which will appear in the Society's Memoirs : — Catalogue of 

 the Birds of Western and North Western Mexico, based "on 

 the collections of Grayson, Xantus and Bischoff; with the 

 notes of Grayson and his biographies of several of the spe- 

 cies. By Geo. IsT. Lawrence. 



Prof. W. H. Niles read an extract from a letter from Mr. 

 A. T. Wing, concerning the continuance of the spontaneous 

 fractures and movements of rock, at the quarry of Mr. W. 

 N. Flynt, at Monson, Mass. 



An account of the character of some of the phenomena at the 

 quarry had been previously communicated to the Society by Prof. 

 Niles, and was published in the Proceedings, Vol. xiv., pp. 80-87. 

 As there stated, the quarry is on a small hill near the village of Mon- 

 son. The rock is gneiss, dipping westward at an angle of about 

 eighty degrees. Joints running nearly parallel to the general sur- 

 face of the hill divide the rock into beds varying in thickness from 

 one inch and a half to several feet. These beds extend for long dis- 

 tances unbroken by any divisional planes, and on that account are 

 remarkably well adapted to manifest any lateral pressure to which 

 they may be subjected. That there is a lateral force exerted pow- 

 erfully in north and south directions is evident from the facts. 

 When portions of the beds are removed by quarrying the force 

 which was originally distributed throughout the extent of the beds is 

 concentrated upon the remaining parts. When the force becomes, 

 by this concentration, greater than the strength of the rock, as is 

 often the case, the rock is broken, often upheaved, and sometimes 

 with violence and attended by explosions. He had requested Mr. 

 Wing to observe and report such additional phenomena as might 

 occur during his absence last year, and the present letter was in 

 accordance therewith. 



" The spontaneous fractures or breaks have been more frequent 

 and on a larger scale during the past summer and autumn, than for 

 any previous season, which is accounted for, I think, by the fact that 

 the quarries have been worked more extensively. One occurred in 

 the latter part of June, in the new quarry on the west side of the ridge, 

 near where you took a sketch of a break, in the autumn of 1871, and 

 again in the spring of 1872. The break extended about two 



