Hawkins — Mineral Occurrences at Princeton, iV". J. 447 



Geology. — There are three principal joint directions, the 

 most important of which, occupied by the major joint series 

 (tension joints), is remarkably constant in direction throughout 

 the area. It widely affects the sedimentary rocks, and is also 

 found on the borders of the intrusive diabase mass of Rocky 

 Hill, an extension of the Palisade sill. Titanium minerals — 

 brookite and ilmenite — are found in these joints, apparently 

 far removed from diabase, together with analcite and barite, 

 whose derivation from the intrusive rocks is indicated by the 

 occurrence of the same minerals in similar joints in the immedi- 

 ate vicinity of the trap rocks at Byram, Hunterdon Co., N. J., 

 and elsewhere. The occurrence of such minerals in the joint 

 cavities of sedimentary rocks, two miles from the nearest vis- 

 ible igneous rock, is worthy of special note. For the produc- 

 tion of some of them, that is probably necessary. The hypothesis 

 is proposed that these major joints were formed very soon after 

 the intrusion of the igneous rock, at the beginning of a tectonic 

 disturbance which widely aifected the Triassic beds. 



Among the species notedj the following call for special 

 description : 



Brookite. — Brilliant crystals, all less than a millimeter in 

 length, but exceedingly well developed. This is the first 

 reported occurrence of brookite in New Jersey. These crys- 

 tals are transparent, showing a wide band of rich blue color in 

 the direction of the brachydome zone, the portions on either 

 side being amber-yellow. A similar blue color has been 

 observed rarely in some octahedrites. 



Certain of the brachydome faces appear frequently as very 

 narrow, bright lines ; this is especially tri\e of the base c, which 

 never yields a good signal. Measurements of some forms, 

 often repeated, in certain cases show a slight deviation from the 

 theoretical angle which proves that the faces are slightly vicinal 

 or " disturbed." Some rounding occurs, especially on the side 

 faces of the brachydome zone. These crystals (see tig. 1) were 

 measured on the Goldschmidt two-circle goniometer, with the 

 following results : 



Angles. 



Forms Letter 



Sym- 

 bol 



Calculated 

 <p p 





XI 1 





XI 2 





XI 3 





Base c 



(001) 



._ 



















__ 







Minute 



r d 



Brachy- •{ . 

 domes j ~ 



(025)* 

 (043) 

 (021) 

 (041)* 





 

 

 



20 42 

 5132 

 62 06 

 75 11 





 

 

 



2142 

 50 02 

 63 02 

 76 31 





 

 







20 41 

 49 56 

 62 32 



76 07 





 

 

 



21 25 

 50 59 

 6158 

 76 19 



Fair 



Prominent 

 Prominent 

 Rounded 



Pina- j b 

 coids ( a 



(010) 

 (100) 





 90 00 



90 00 

 90 00 







90 00 





 90 00 



90 00 



88 12 







90 00 



Rounded 

 Minute 



i l 

 Prisms ■< L 



( P 



(210) 



(230)* 



(120) 



67 10 

 38 23 

 30 43 



90 00 

 90 00 

 90 00 



67 30 

 39 00 



90 00 

 90 00 



66 22 

 38 12 



90 00 

 90 00 



30 23 



90"00 



Prominent 



Fair 



Fair 



Pyramid X 



? (313)* 



74 20 



49 22 



__ 



__ 



75 18 



49 00 



.. 



_. 



Prominent 



