Assodaiion of American Geologists and Naturalists. 103 



quire a polarity contrary to that of the earth, thus diminishing in a sen- 

 sible degree the effect of terrestrial magnetism in the neighborhood. 



Prof. Locke also announced the fact that the point of greatest mag- 

 netic force is at or near Lake Superior. Thus there are three impor- 

 tant poles or points nearly on the same meridian, nearly equally distant 

 from each other, and directly north of the United States, being between 

 the longitudes 85 to 90 : first, the true north or astronomic pole ; sec- 

 ond, Ross's pole of perpendicular dip, and of magnetical convergence, 

 twenty degrees south of the astronomic pole ; third, the pole of maxi- 

 mum intensity of magnetic force, 21^° south of Ross's pole, ^2^/ south 

 of the north pole, viz. in 47J° of noi'th latitude. 



Prof. L. added that he had made a journey to Europe and purchased 

 the instruments for this research at his own cost ; that he had travelled 

 many thousands of miles in making the investigations, and mostly at his 

 own cost ; that now he has developed so many points of what seems to 

 him to be a regular system of magnetism in the United States, that he 

 feels almost irresistibly impelled to prosecute the subject, but the econ- 

 omy of his domestic affairs renders it inconsistent with his duty. 



Prof. Johnson presented an invitation from Lieut. Gilliss to 

 visit the Naval Observatory this afternoon at 5 o'clock, which 

 was accepted. 



The President presented the invitation of Capt. Wilkes to 

 visit the collections of the Exploring Expedition at the Patent 

 Office, and also to visit him at his residence on Saturday eve- 

 ning, which was accepted. 



Prof Hitchcock read a paper on " the Trap Tufa, or Volcanic 

 Grit of the valley of the Connecticut River, with inferences as 

 to the relative age of the Trap and Sandstone," with illustrations. 



The trap tufa is a peculiar rock of volcanic origin, differing from 

 common trap in being conglomerated, or taking round pebbles into its 

 composition, and being stratified. It also contains one marked exam- 

 ple of a vegetable petrifaction, appearing like organic remains in igne- 

 ous rock. But Prof. H. endeavored to show that it was produced before 

 the main ridges of trap along Connecticut river, by precursory outbursts 

 of pumice, scoria, ashes and melted matter, flowing over the bottom of 

 the then ocean, and mixing with the sand and gravel, and so becoming 

 more or less stratified, and enveloping animals and plants. After this, 

 layers of sandstone accumulated over it, and finally the main ridges of 

 trap were protruded through the strata, tilting them up. Hence this 

 rock, constituting East and West Rock, and the Hanging Hills in Con- 

 necticut, and Holyoke and Tom in Massachusetts, was the one of latest 

 origin in that valley — a point which had not before been determined. 



