GKOLOGY. 285 



horizontal, to be hereafter cemented together as a stratum of conglomerate 

 rock, would not the argument founded on their shape and position be even 

 stronger than in the case of the Newport conglomerate? Yet nothing is 

 more certain than that they owe their shape and arrangement to the peculiar 

 movement and attrition to which they have been subjected by the action 

 of the waves. 



Thus, as regards the Newport rocks, and most other conglomerates which 

 had fallen under his notice, Prof. Rogers saw no difficulty in referring the 

 form and arrangement of the pebbles to the familiar agencies above indi- 

 cated. He did not, however, doubt that, in some highly metamorphic 

 districts, conglomerate rocks were to be found which had sustained great 

 internal changes through the effects of heat, chemical action, and violent 

 pressure. Such he has long thought must have been the conditions in 

 some parts of the Blue Ridge and South Mountain chain in the Middle 

 States, and such, perhaps, were the influences which operated on the gneis- 

 soid conglomerates of the Green Mountains, to which Prof. Hitchcock has 

 referred in his communication to the Society. 



ON THE FORMATION OF TRAP DIKES. 



At the meeting of the American Association, 1800, Mr. J. D. TVhitnev 



i 



read a paper, prepared by himself and Col. Foster, on the origin and strati- 

 graphical relations of the trappean rocks of Lake Superior. It was a 

 minute description and discussion of the traps found in the Lake Superior 

 region, especially about the copper mines at Keweenaw Point, and pre- 

 sented as many objections as possible to the theory, now pressed with much 

 vigor, that trap is not of igneous origin. 



Prof. Agassiz quite concurred with the authors of the paper, that an 

 examination of the shores of Lake Superior fully established the igneous 

 origin of trap. The evidence of the heated mass upon the .sandstone below 

 was as plain as that of a hot poker upon wood. He thought that if the 

 advocates of the aqueous origin of the trap would examine some of these 

 places, they would be convinced that they were wrong. 



Prof. "Win. B. Rogers coincided in maintaining the igneous origin of trap, 

 and adduced some instances supporting that theory. 



Prof. Agassiz said that he had observed the influence of the rocks upon 

 the dikes, as well as the influence of the dikes upon the rocks. There was 

 a, very good instance of this at Nahant, where the influence of the rock in 

 producing a slow cooling of the hornblende was seen in the very large 

 crystals there found. 



ON THE TEMPERATURE OF THE EARTH AT GREAT DEPTHS. 



At a recent meeting of the Boston Society of Natural History, Prof. W. 

 B. Rogers remarked, that in reference to the increase of temperature at 

 groat depths, as a means of determining the thickness of the solid crust or 

 shell of the globe, much uncertainty must attend such calculations until all 

 the necessary data have been ascertained. It is not merely requisite to 

 know the lav/ according to which the temperature augments as we descent, 

 and the ordinary melting point of the different rocky material^ forming the 

 cru^t, but we must ascertain how and In what degree the nit.-itin_ r point in 



