a 
il 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 pperosthed, 
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 
ol water. 2 
Dr. R. EF. Rogers called the attention of the Association to the 
subject of limestones, observing that he thought a 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, alarger 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 cc bined 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, consisting 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. 
Fis 
