300 J. F. Kemp — Basic Dike near Hamburg. N. J. 



ing map, figure 1, illustrates the general geology and geogra- 

 phy. Referring to the squares, the syenite occurs in Bl, B2 and 

 A2. It lies along the contact of the Kittatinny (Oriskany) con- 

 glomerate, and the Hudson River slates. The older described, 

 basic rocks are all in the slate and outcrop in A2, B2, and C2. 

 The dike here described is in undetermined, but probably 

 lower Silurian, blue limestone, in D-t. The Franklin Furnace 

 dike is in white limestone in F4. Several initial letters are 

 inserted to mark the towns : L, in A3, is Libertyville ; B, in 

 C2, is Beemerville; D in C4, is Deckertown ; H, in D4, is 

 Hamburg ; F, F, in F4, is Franklin Furnace. The basic dikes 

 appear at once to be in a general northwest and southeast ar- 

 rangement, and it is natural to infer that they represent a series 

 of outcrops along a common line of emergence. The Ham- 

 burg dike is on the farm of Lewis Havens (lately Zephaniah 

 Havens) and the farm of Miles Harden lies just over the fence. 

 A roadway runs near it. The dike cuts blue cherty limestone, 

 which strikes N. 60 E. and dips 50 N., while the dike itself 

 strikes N\ 25 W., being nearly at right angles with the lime- 

 stone. There are small veins of quartz along the contact, but 

 otherwise no apparent metamorphism. The width of the trap 

 is 15 to 20 ft. and the outcrop is about 50 yards long. 



In the hand specimen, it is a dark rock, extremely tough, 

 and thickly set with biotite. In portions some curious, sphe- 

 roidal inclusions appear, which form one of the chief points of 

 interest. They vary up to 10 mm in diameter and are filled 

 with white minerals. They can be cracked out of the matrix, 

 because they are surrounded by concentric scales of biotite. 

 They attracted attention at once in the field and were brought 

 to the laboratory as one of the features deserving special atten- 

 tion. 



In thin section the dike rock is seen to be made up of biotite 

 and pyroxene as its larger minerals, and these are set in a very 

 feebly refracting, or entirely isotropic groundmass, that is 

 chiefly analcite. Throughout the groundmass are abundant 

 gegirine rods, apatite needles, magnetite and occasional plagio- 

 clase. In places titanite crystals are extremely numerous and 

 the magnetite likewise shows the usual evidence of being titan- 

 iferous. Figure 2 is a drawing of the rock, the outlines of 

 whose crystals were traced with a camera lucida. The endeavor 

 was made to give as characteristic a portion as possible. The 

 specific gravity of the rock is 3*040 to 3 - 019 as determined on 

 several specimens. 



The biotite is the deep brown variety characteristic of the 

 nepheline rocks. It is nearly uniaxial and as noted by Profes- 

 sor Rosenbusch (to whom some specimens of the rock were 

 sent and to whom the writer is indebted for a very kind letter) 



