Association of American Geologists. 171 



Peter A. Browne, Esq. laid on the table, for the inspection of 

 the members, a suite of specimens, chiefly fossils, from the chalk 

 basin of Paris, collected and labelled by A. Brongniart. 



Prof. Locke made some observations concerning the connec- 

 tion of magnetism with geology, mentioning an instance where 

 he found an increase of the dip and intensity as he approached, 

 from south to north, a certain point or meridian line, and a de- 

 crease as he receded from it ; also remarking that a similar change 

 is found upon crossing the Ohio river : querying from this, whether 

 the water of large streams running east and west, has an influ- 

 ence on the magnetic relation. 



Dr. Houghton remarked, that in the vicinity of the great 

 northwestern lakes a change in the magnetic deflection was fre- 

 quently found on approaching within a few miles of a large body 

 of water. 



Dr. R. E. Rogers called the attention of the Association to the 

 subject of limestones, observing that he thought the magnesian 

 character of these rocks generally had not received sufficient atten- 

 tion. He stated that he had found, upon analyzing some of the 

 lower limestones of Pennsylvania, a larger proportion of magnesia 

 than is requisite for the formation of a true dolomite, and threw out 

 the query as a point of scientific interest, whether the carbonate 

 of lime and carbonate of magnesia were chemically combined in 

 the proportions to form dolomite, and this mingled throughout 

 the excess of the carbonate which might be present, or whether 

 the two carbonates were mechanically and uniformly intermin- 

 gled. 



Dr. Jackson stated, that he considered the granular or crystal- 

 lized dolomite to be a regular chemical double salt, consistmg of 

 one equivalent of carbonate of lime and one equivalent of car- 

 bonate of magnesia. But he had never found any magnesian 

 limestone to contain more than this proportion of magnesia, al- 

 though he had frequently analyzed limestones containing a less 

 proportion than one equivalent of magnesia. His published anal- 

 yses will illustrate this remark. 



Dr. J. inquired whether Dr. Rogers had ascertained if the 

 limestones to which he alluded did not contain the hydrate or 

 silicate of magnesia, mixed with dolomite. If the rock was of 

 the compact variety, this might have been the case. 



