AMERICAN GEOLOGISTS AND NATURALISTS. 25 



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 other 

 formations ; this observation was not intended to apply to the 

 joints of slate rocks. 



Dr. Jackson cited the joints or fractm-es 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 brolcen 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 Michigan 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 INlichigan 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, seeming 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 

 elucidate the present subject. Specular iron was generally ob- 

 servable among the interstices of the pebbles at Purgatory, and 

 more or less of iron and lead ore was generally to be found at 

 the juncture and fissures of the trap dykes. 



Prof. Hitchcock thought that the steps of the new red sand- 

 stone of the Connecticut valley were the result of the fractures 

 referred to by Prof. Mather — they were nearly coincident with 

 the strike of the strata, as if caused by elevatory movements. He 

 found difficulty in distinguishing between fissures produced by 

 mechanical violence and joints properly so called; he viewed 

 those of the conglomerate as mechanical, those of the slates as 

 chemical. Two cases occurred to him as worthy of notice ; the 

 first was of a dyke of gi'eenstone crossed by parallel ti-ansverse 

 planes two or three feet apart and at right angles to the strike of 

 the vein. The second case seemed to throw^ some light on the 

 origin of this class of phenomena; it occurred in a bed of the 

 common blue diluvial clay — the horizontal layers were unmoved, 

 but some of them were divided into double rhombs. The ex- 



