GLACIO-AQUEOUS ACTION IN NORTH AMERICA. 211 



I must not omit to mention, however, that Dr. Jackson, in his 

 report on the Geology of New Hampshire, gives an entirely- 

 different explanation of this case ; referring the fractm-e of the 

 laminae to " a crossing and overlapping of the strata, forced out 

 from their original position by a lateral thrust, which may have 

 been effected by the elevation of the neighboring and subjacent 

 primary unstratified rocks." — ( Geology of N. Hamp. p. 57.) 



Soon after noticing this opinion, I revisited the locality, and 

 made a careful examination of it with reference to those views ; 

 and I must say that I could not discover the least evidence of any 

 disturbance of the strata beneath the fractured laminEe ; nor of 

 any crossing and overlapping of the strata ; nor of a lateral thrust ; 

 nor of unstratified rocks within several miles of the spot. If the 

 figure accompanying Dr. Jackson's account was intended to rep- 

 resent the quarry, (of which I am not quite certain, from his lan- 

 guage,) I must observe, that I could not discover, at the quarry, 

 the thick mass of slate represented in his figure as lying in a 

 northeast and southwest direction, and pressing against the frac- 

 tured laminae. The face of the quarry appears at first as if only 

 the middle part of it had been subject to the crushing agency : but 

 occasional fragments of the slate seen along the top of the quarry, 

 beneath the soil, render it probable that originally the whole top, 

 certainly aj^ the northern part of the hill, was fractured in a simi- 

 lar manner to the part now exposed near the middle, and that 

 the fragments have been swept away from the other parts. 



I have thus frankly expressed my views of this case, not from 

 a desire to enter into any controversy on the subject, but in the 

 hope, that since Dr. Jackson and myself honestly differ about it, 

 other geologists may be induced to visit the spot, in order to de- 

 cide which of us, or whether either of us, has given a right ex- 

 plication of it. Such differences of opinion must be expected 

 in geological investigations : but they will do no harm, if the 

 opposite opinions be maintained with a proper spirit ; with a 

 readiness to abandon them when proved to be false. 



Dr. S. L. Dana has pointed out to me a similar case of a 

 fractured ledge of gneissoid rock, near one of the factories in the 

 city of Lowell. It rises but a little above the surface of the 



