172 Association of American Geologists. 



Dr. James Rogers thought we must consider dolomite as a 

 true double salt : 1 atom carbonate of lime 4- 1 atom carbonate of 

 magnesia ; the excess of magnesia found in our limestones must 

 be considered a mechanical mixture. 



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

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

 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 sec- 

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

 prevailing ; the first had a general direction of north by northeast, 

 the second set were nearly perpendicular to the former — besides 

 these, there were two other sets not so well defined. The joints 

 in the primary were not so smooth and well marked as in oiher 

 formations ; this observation was not intended to apply to the 

 joints of slate rocks. 



Dr. Jackso7i cited the joints or fractures of the conglomerate 

 around Boston, and particularly at Roxbury, Mass., and also in 

 the island of Rhode Island, at a place called Purgatory, the large 

 pebbles are broken by these fractures, without dislocation or loos- 

 ening from their beds. He supposed the parallel and uniform 

 cracks in the lime rocks and slates of that country to have con- 

 nection with the different epochs of irruption of the trap, granite 

 and porphyry. 



Dr. Douglass Houghton inquired of Dr. Jackson if these 

 cracks in the conglomerate had reference to the line of bearing, 

 remarking that in Michigan they were nearly at right angles to 

 the line of the longer diameter of the pebbles. Dr. Jackson re- 

 plied that such was the case in the cases he had cited ; that at 

 Purgatory the pebbles were very large, ovate, and arranged with 

 their longer diameters in one direction, and seemed to be joined 

 together by very little cement, and yet they were broken at right 

 angles to their longer diameter, without dislocation. He stated 

 that Mr. A. A. Hayes had found that chloride of calcium would 

 concrete pebbles of quartz into a firm mass — this fact might elu- 

 cidate the present subject. Specular iron was generally observ- 

 able among the interstices of the pebbles at Purgatory, and more 

 or less of iron and lead ore was generally to be found at the junc- 

 ture and fissures of the trap dykes. 



