168 ANNALS NEW YORK ACADEMY OF SCIENCES 



grees east, even at B3a-am. North of the Rocky Hill trap sill this joint 

 series appears, in the shales, and in the borders of the intrusive diabase, 

 but apparently not within its central portion, where the joints are curv- 

 ing and irregular. The lines of major jointing show a tendency toward 

 verticality, although they sometimes hade about ten degrees east. It is to 

 be noted that this vertical position has little significance, as the joints 

 have probably been tilted more or less since formation. Moreover, a 

 knowledge of the precise direction of this or any other joint series, as 

 pointed out by Leith, and of the direction in which its walls appear to 

 have moved during tension, does not fully inform us as to the real direc- 

 tion of the force which produced the tension. "It may not be certainly 

 determined what combination of stress and strain conditions have been 

 present throughout the development of a given cleavage, although the 

 relations of cleavage to the final total strain may be known," as he re- 

 marks on page 113 of his bulletin. All that we can safely say, then, is 

 that the tension occurred in a nortliwest and southeast direction in this 

 part of the Lockatong belt. This tension acted in some cases from one 

 side of the joint series, and again from the other. 



In explanation of the production of this major joint series in the 

 Lockatong formation, and of the minerals therein contained, the writer 

 advances the following hypothesis : 



The occurrence of titanium minerals in the fillings of these joint fis- 

 sures is one of the most interesting and important features. Titanium, 

 while abundant in disseminated condition in many rocks, is seldom seg- 

 regated in one place. Brookite and ilmenite are found in our mineralized 

 zones, and while they are present only in small amount, are of very beau- 

 tiful development. Ilmenite has been found at Princeton, in tension 

 joints and breccias. Ilmenite of exactly the same habit and of closely 

 similar form has been found at Byram, occupying tension joints, demon- 

 strably similar in nature and method of production to those of the above 

 locality, and in this latter case only fifty feet from an intrusive diabase 

 sill, so close to the trap that its derivation from the latter would seem 

 certain, in view of large quantities of ilmenite w^hich are known to form 

 part of the normal constitution of the trap, and of the analcite and other 

 zeolites in which the ilmenite is implanted. Ilmenite has also been dis- 

 covered close to the diabase at New Hope, Bucks Co., Pa. It is embedded 

 in tourmaline crystals in the baked hornfels above the trap, and some 

 much altered sandstone layers there are at times quite highly charged 

 with it. 



The tourmalines in the homfels were produced by the expulsion of bare 

 acid solutions from the trap while it was crv'stallizing, as explained uy 



