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of Trap, varying from a few feet to several hundred feet in 

 breadth, of which, and of the adjacent sedimentary rocks, some 

 good sections have recently been disclosed by the railroad cut- 

 tings. These show a nearly vertical position of the clefts through 

 which the igneous matter has been erupted, and indicate their 

 general direction to have been a little east of north. 



The strata penetrated by the Trap are nearly horizontal, and 

 appear to have suffered no considerable tilting or other disturb- 

 ance of position from the intruded igneous mass. It would seem 

 that, prior to the ingress of the molten matter, numerous nearly 

 parallel cracks were formed in the horizontal strata, which were 

 then filled from beneath without much further displacement. 



In the vicinity of the dykes, the shales and sandstones present 

 various degrees and kinds of alteration. The red shales are 

 transformed into a dark purple or bluish gray semi-crystalline 

 mass, often of flinty hardness, and greatly increased density. 

 These are specked with shot-like forms, or interspersed with 

 larger rounded segregations, including crystalline minerals more 

 or less perfectly developed, and presenting the aspect of certain 

 kinds of amygdaloid. The sandstones are greatly indurated, and, 

 in some instances, the more sandy shales, where touching the 

 Trap, are converted into a close-grained, milky quartz, which 

 forms a narrow ribband on each side of the dyke. These great 

 alterations in the mineral characters of the strata are, however, 

 not traceable far, in a horizontal direction, from the walls of the 

 Trap dykes. Often within twenty feet, the horizontal rocks dis- 

 play their usual characters almost unchanged. 



The most remarkable exhibitions of igneous influence observed 

 in this particular district, were traced over wide spaces of the 

 sedimentary rock, in which no dykes were seen. In these locali- 

 ties, the altered beds lying horizontally are in some places cov- 

 ered by disintegrated Trap, forming a coarse gray Trappean sand, 

 which, followed to some distance, is observed to pass into masses 

 of globular and stratified Trap, evidently overlying the other 

 rocks, at a level corresponding to that at which the altered sur- 

 face is seen exposed. From this fact, and the observed more 

 rapid disintegration of the Trap than the adjacent altered rocks. 

 Prof. Rosers concluded that much of the surface now uncovered 



