HAWKINS, LOCKATOSG FORMATION OF THE TRIA88IC 163 



cycle. Hence it is suggested that the Lockatong may mark the close of 

 the erosion cycle which was begun when the Stockton beds were deposited, 

 and that the Brunswick may indicate the beginning of a second cycle, 

 probably brought about by a slight upward warping of some of the land 

 surrounding the basin, which cycle continued tlirough Brunswick time. 



Crystal Growths 



Much of the Lockatong argillite, even in its densest phases, is charged 

 with disseminated specks of crystalline calcite, usually minute in size. 

 This calcite occurs as often in the red rocks as in the gray, and is prob- 

 ably a secondary filling in spaces from Avhich some earlier material has 

 been leached. It often follows the course of a horizontal stratum, and 

 does not favor the joints. 



Apart from the foregoing, there are other growths of a somewhat simi- 

 lar nature, but of more limited distribution, wliicli, on detailed examina- 

 tion, are found to exhibit certain systematic ])ec'uliarities that suggest a 

 somewhat more complicated mode of origin. These are curious fan- 

 shaped radiations of white material, which appear most plainly in the 

 quarries at the east end of Princeton, being at times quite conspicuous 

 features. The arrangement, where most typically exposed, is as follows : 

 Filling one stratum, perhaps two feet thick, are sharply defined, slightly 

 tapering lines of white material, which originate at a common horizon 

 defined with great clearness. The white stringers diverge downward 

 from this horizon in slightly curving lines, meeting at the top in points 

 which are definitely spaced, according to the amount of development of 

 the whole. Vertical cross sections of these radiations in different direc- 

 tions, as well as vertical sections of the same portions at right angles to 

 each other, afforded by the corners of joint blocks, show that what we 

 really have is a series of conical arrangements, composed of strings of 

 crystals which radiate downward always. Above the layer of conical de- 

 velopment there is usually a zone of a foot or less containing white crystal 

 grains in irregular arrangement, as if considerably disturbed. 



Thin sections of these crystals under the microscope show that tliese 

 are indeed cr\^stal cavities, the appearance of regular outlines in the hand 

 specimen being amply borne out by tlic appearance in the sections. The 

 cavities at times show very definite angles which outline a form that 

 appears to be of a monoclinic or triclinic type. 



In these exposures, tlie crystal groups follow definite horizons in argil- 

 lites which are gray, while at Scudders Falls (Traction Oompany^s 

 quariy, on the Pennsylvania side of the Delaware River) one massive 

 stratum with a strong reddisli bi-own color sliows a typical development 



