164 ANNALS NEW YORK ACADEMY OF SCIfJNCES 



of the same arrangement, although it is not so well deveh)ped as in the 

 Princeton quarries. 



It is evident that the material now filling these cavities is not original. 

 This filling is composed of an outer lining of isotropic analcite, which 

 has coated the walls of each cavity with free crystals whose outlines 

 plainly show, and calcite, w^iich has filled in the balance of the cavity. 

 It appears that the entire rock mass has been soaked through and through 

 with solutions bearing the same minerals, which are found in excellent 

 development in the larger cavities of the joints. These solutions, as else- 

 where demonstrated, had their origin with the intrusive rocks not far 

 distant. 



The exact nature of the original mineral that grew here in the Ijocka- 

 tong muds could not be determined from the material available. Some 

 of the crystal cavities occupy positions within the fillings of mud-cracked 

 layers, showing that they grew after the filling of the cracks by deposition 

 of another layer of sediment above. The whole system of conical crystal 

 growths is most unusual. Its similarity to the well known cone-in-cone 

 structure seems only apparent. There is no tendency in the argillites of 

 the Lockatong to break along the lines of crystals; breakage takes place 

 across the cones, along the bedding, showing the crystals following a pat- 

 tern of wavy lines within the cone. A thin section of cone-in-cone lime- 

 stone from Erie, Pa., showed no traces of crystal growths along the lines 

 of parting, whose production appears to have been due to pressure alone. 

 Possibly the growths in the Lockatong followed cracks which had been 

 produced by pressure ; but it is hard to explain by this method the growth 

 of isolated cones at regular intervals, whose production would require 

 isolated points of intense pressure, similarly spaced. It is more likely 

 that the original mineral was something similar to gypsum, which in its 

 crystallization often follows a radiating habit, and which grew in the mud 

 before induration. 



Evidently the animals and plants of Lockatong time, carbon from 

 whose remains so often causes dark colors in these rocks, existed in the 

 general region where they are now found, since ganoid scales in certain 

 localities, as at Phoenixville, are still clinging tightly together. Plants 

 require some moisture, and estheriae still more, while fish remains indi- 

 cate considerable bodies of Avater — playa lakes — of some extent. Dark 

 shales of this character, found in the proximity of limestone layers and 

 carrying fish scales and estheria?, never have been observed to show mud- 

 cracks, a fact which would indicate that the sediments were constantly 

 covered with water and out of contact with the air. Tlie passage into 

 gray argillites above, in which there is only a trace of carbon, and thence 



