6 4 



SCIENCE. 



the same formation exposed so abundantly on Manhattan 

 Island. 



Associated with the crystalline Archaean rocks that to 

 a limited extent border Hudson County on the east, are 

 beds of quartzite and serpentine, exposed in the bluff 

 known as Castle Point at Hobcken. This premontory 

 is about thirty acres in area, and is limited on the east 

 by bold bluffs of serpentine. The rock here exhibits 

 considerable variety, being sometimes yellowish and dull 

 in appearance, ana so earthy as to crumble between the 

 fingers ; again it is compact, dark green in color, and 

 furnishes an ornamental, although interior, building stone. 

 This rock is a silicate of magnesia containing chrome- 

 iron in scattered grains, and lurnishes also the minerals 

 marmolite, brucite, nemalite and magnesiie. 



The quartzite or jasperoid reck, occurring on the 

 souihern slope of the serpentine, in the neighborhood of 

 the Stevens Institute, has, together with ihe serpentine, 

 been referred to the Archaean series, but as the exposures 

 are now obliterated little can be said concerning it. 



TRIASSIC ROCKS. 



In Hudson county we have a portion of the eastern 

 border of the Triassic formation which forms a band 

 thirty miles broad across the Stale. In general with the 

 Triassic formation in New Jersey and the Connecticut 

 Valley, the rocks are here lelspaihic sandstones, slates 

 and shales traversed by sheets and dikes of trap. The 

 sedimentary rocks occupy nearly the whole area of the 

 county and dip uniformly to the northwest at an angle 

 of about 15". The sandstone is largely composed of 

 granules or fragments of felspar, cemented by oxide of 

 iron to which the reddish or brownish color ot the rock 

 is due; this is the stone so largely used for architectual 

 purposes in New York and me neighboring cities. 

 Traversing these inclined beds of sedimentary rocks, and 

 in a general way conformable with them, are sheets of 

 intrusive trap, which now owing to unequal erosion, 

 form the most prominent features in the topography of 

 the county. This statement holds gocd, aiso, for the 

 entire Triassic area in New Jersey, anu with more or less 

 accuracy for this formation 111 general along the Atlantic 

 slope. The main trap ridge in Hudson County, com- 

 posing the highland known in different portions ot its 

 course as Bergen Hill, Jersey City Heights, and the 

 Heights ot VVeehawken, is cominued northward with 

 increasing height, and forms the bold picturesque shore 

 of the Hudson as far northward as Havcrstraw. The 

 outcropping edge of the trap, especially in Hudson County, 

 has been abraded by glacial action so as to form an 

 irregular, baaly drained, plane surface. Although in a 

 general way lullowing the bedding of the associated 

 slates and sandstone, the trap.sheet is really unconloim- 

 able to them and breaks across their bedding in various 

 places. From both the upper and lower sunaces of the 

 main trap sheet smaller sheets and dikes of molten rock 

 have been intruded among the stratified beds. Examples 

 of these branches lrom the principal mass may be seen 

 at the base of the cliffs along the west bank of the Hud- 

 son, from Hoboken northward. Secondary sheets origi- 

 nating from the upper surface also appear on the west- 

 ern border of Bergen Hill, where they have been 

 accented by erosion. The rntrusive nature of the trap 

 sheets and dikes is shown by their crystalline structuie, 

 their unconformity to the inclosing strat.fied beds, and 

 by the metamorphism produced in the strata with wh.ch 

 they have come in contact. A section exposed in the 

 cliffs bordering the Hudson a few miles north of 

 Hoboken, is given in the following figure, and illustrates 

 especially the abrupt manner in which the New Jersey 

 Triassic area is cut off along its eastern border. 



In the diagram D represents the sheet of drift that 

 covers the eroded surface ol the hill, and S the slates that 

 unconformably underlie the trap into which a small 



Fic. 2— Section at Dog's Point, Weehawken. 



secondary sheet of the crystalline rock has been in- 

 tiuded. Beneath the slates are beds of light colored 

 telspathic sandstone ending in a cliff at the water's edge ; 

 the whole series has the usual dip of 15 N. W. 



The irregular line formed by the eastern boundary of 

 the trap is caused, at least in two instances, by sheets ot 

 trap that leave the main mass at an angle and stand out 

 in ridges tangent to the principal line ot cliffs ; examples 

 of this feature, which is difficult ot desenptio n w ithou 

 illustrations, may be seen at Kings Point, Wehawken 

 and Fairmont Hill, Jersey City. 



POST-TRIASSIC HISTORY. 



No records are found in Hudson County, of the Jur- 

 assic cretaceous or Tertiary periods during all these geo- 

 logical ages ; the area under discussion must have been a 

 laud surlace exposed to subaenal denudation, 'ihe same 

 destructive agencies were at work, too, with accelerated 

 energy during the Quaternary period. The result is that 

 we have but a decimal portion of the original Triassic 

 formation remaining. 



During the Quaternary, northern New Jersey, in com- 

 mon with a gieat area in the northeastern- part of the 

 continent, was buned beneath glaciers ot great thickness. 

 In Hudson County the ice-sheet moved from north-west 

 to south-east, ploughing out in its journey the soft Trias- 

 sic shales and sandstones, and grinding off the projecting 

 ridges ot trap with a lorce so inesistable that a mountain- 

 ridge like the Palisade range could not deflect it from its 

 course. When the climate ameliorated and the glaciers 

 retreated northward, the hard crystalline trap was left 

 wrth a pohst.ed surface that glitters in the sun light, ar.d 

 is crossed by deeply engraved lines that faithfully record 

 the direction from which the glaciers came. In places 

 tne rock rises into smootnly rounded hillocks, lorming 

 typical roches muutonnees. The load that the glaciers 

 carried was spread over valleys and uplands, lorming a 

 continuous sheet ot glacial drift, which now composes 

 the immediate surface ot a large portion ot the county. 

 This glacial dntt is generally a tenacious clayey deposit, 

 at times fifteen or twenty leet thick, ot a reddish color, 

 derived from the debris of Triassic shalas and sandstones 

 that enter largly into its compositton. Scattered 

 through it are ooulders of trap, sandstone, slate, etc., 

 thai have been transposed but a short distance, and 

 others ot gneiss and conglomerate, the parent ledges of 

 which are thirty or fony miles to the northwestward. 

 Hudson county also furnishes examples ot modified 

 drilt, consisting of irregular layers ot sand, gravel and 

 small boulders, all well rounded and plainly assorted and 

 deposited througn the agency ot running water. 



More recent than the glacial deposits are the sand 

 dunes that skirt the base ot the upland on all sides. 

 Again more recent ihan the hills ot atolian sand are 

 the many deposits of peat and mud still in process of 

 formation along nearly the whole water-front of the 

 county. 



'IJie bed-rock in Hudson county is, in most places, ex- 

 cept. ng on the uplands, deeply covered by Quarternary 

 and recent deposits. The topography ot the rocky floor 

 of the county and of the neighboring portions ot New 



