24 PROCEEDINGS OF THE ASSOCIATION OF 



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 intiuence on the magnetic relation. 



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

 northwestern lakes a change in the magnetic deflection was fi-e- 

 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 at- 

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

 the lower limestones of Pennsylvania, a larger proportion of mag- 

 nesia 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 

 intermingled. 



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, 

 although he had frequently analyzed limestones containing a 

 less proportion than one equivalent of magnesia. His published 

 analyses 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. 



Dr. .Tames Rogers thought we must consider dolomite as a 

 true double salt — 1 atom carbonate of lime + 1 atom carbonate 

 of magnesia ; the excess of magnesia found in our limestones 

 must be considered a mechanical mLxture. 



A communication was received from Peter A. Browne, Esq., 

 expressing a willingness to read before the Association "an Es- 

 say on Aerolites, or Meteoric Stones," at the next annual session. 

 Laid upon the table. 



Prof. Mather made a verbal communication on the joints in 

 rocks, particularly as they occur in the primary, transition and 

 secondary of this country. He found two principal sets of joints 



