SPARRY LIMESTONE. 73 



directions. It is not a pure limestone, though it is very frequently employed for lime. 

 If this rock should prove sound after it has heen penetrated a few feet, it would make a 

 fine and beautiful marble similar in aspect to the so-called " Egyptian marble." 



The dip of this rock is conformable to that of the. whole system, varying from east to 

 southeast. The strike corresponds also to the other members of the system, which varies 

 but little in any part of its range from N. 10° E. In the State of New-York, the Sparry 

 limestone occupies the belt of country comprising the eastern part of Dutchess, Columbia, 

 Rensselaer and Washington counties, passing about one mile east of Hoosic four-corners ; 

 and in its progress north, it strikes the west line of Arlington in Vermont. It forms the 

 eastern boundary of the Taconic slate. It passes through a region of rounded hills, steeper 

 upon their western than upon their eastern sides, but less elevated than the range still 

 farther east, near whose bases the Stockbridge limestone ranges. It is not an easy matter 

 to trace this rock continuously, partly from the great amount of debris which has been 

 deposited upon it and the adjacent masses. In some parts of the range it seems to have 

 been engulphed or pinched out, or lost by some of the disturbances to which the rocks 

 have been subjected. Still it is a very persistent mass, and is never lost in the whole range 

 through New-York but for small distances. This rock is the Transition limestone upon the 

 geological map constructed by Prof. Dewey for illustrating the geology of Berkshire 

 county, Massachusetts. The tunnel of the Western Railroad is cut through this rock, the 

 length of which is about five hundred feet. At this place, the junction of the slate and 

 limestone is by intermediate beds of plumbaginous slate, as it is termed, a variety which 

 soils the fingers. 



It only requires an examination with the microscope to satisfy any one that the dark 

 colored strata are highly charged with sulphuret of iron in fine crystals, which, on exposure, 

 decompose and form a thin coating of a dark material, the greater portion of which is 

 sulphur, mixed probably with the black oxide of iron. According to this view, there is 

 no important change produced by heat at the line of contact of the rocks, but simply a 

 decomposition of the sulphuret of iron disseminated through the intermediate layers. At 

 the Tunnel, too, as in many other places, the limestone is highly siliceous ; and so com- 

 mon is this earth, that but few localities furnish a stone suitable for lime. I have no 

 satisfactory account to give of the origin of the calcareous veins : their presence is constant, 

 though not always equally profuse. 



The thick mass which I have had in view, and which ranges through the eastern part 

 of the counties named, is not the only mass of limestone which possesses this character. 

 Proceeding westward from this point to the western bounds of the Taconic slate, thin 

 deposits of a sparry limestone occasionally occur, some of which are twenty feet thick, 

 and others less ; the thinnest and most unimportant being farthest towards the western 

 bounds of the slate. The causes, therefore, which concurred in the production of the 

 Sparry limestone, continued to operate with more or less energy, but only at intervals, 

 during the whole period when the slate was being deposited. I do not suppose, however, 

 that all the beds of this character are different beds : some are undoubtedly mere repeti- 

 [Agricultural Report.] 10 



