HAWKINS, LOCKATONG FORMATION OF THE TRIAS8IC 173 



shown in the quarries, where rock tliat has been immersed in water for 

 long periods will be found upon breaking to be perfectly dry inside, with 

 the exception of an eighth of an inch next to the surface. The small 

 amount of pore space is caused by the extreme fineness of grain and by 

 the siliceous cement with which the rock is filled. Since water is unable 

 to gain access to the interior, the damage to the rock by frost is scarcely 

 perceptible. The effects of expansion and contraction with changes of 

 temperature are also slight. 



The argillites will melt at about 1100° Centigrade. It is not safe, 

 therefore, to employ them for structural material in chimney linings or 

 in other places where they may be exposed to intense heat. 



The argillite is evidently a very strong and resistant rock, and it has 

 successfully stood the test of the last one hundred years in such buildings 

 as were constructed of it. On account of its varied colors, the argillite 

 has been used with pleasing results in the construction of some of the 

 newer dormitories at Princeton University. It is being extensively em- 

 ployed in the new Graduate College connected with the university, and in 

 the new High School building at Princeton. Its use in concrete con- 

 struction has been extensive in the Graduate College; it seems to sen-e 

 the purpose well, except that the argillite fragments are sometimes too 

 smooth to offer a good surface for firm adhesion of the cement. There 

 seems to be no reason for believing that this rock is not as good as trap 

 rock for many of the purposes for which the latter is usually employed ; 

 the trap rock, moreover, often has the disadvantage of much greater 

 coarseness, and a corresponding degree of susceptibility to the destructive 

 action of the weather. The argillite, as a building material, should be 

 more widely known. 



Summary 



The Lockatong formation is the middle member of the Newark series 

 of Triassic rocks, extending from a point just west of Phoenixville to the 

 vicinity of Princeton. 



The rocks constituting the formation are dense, fine-grained, massive 

 mud-rocks known as argillites, with some shales. The formation as a 

 whole has a decidedly lens-like character. The present investigations 

 have led us to believe that these sediments were laid down in an inland 

 basin, and probably in the center of that basin. This h)rpothesis is sup- 

 ported by the general structure as observed in the field, by the testimony 

 of the fossil estheriae, fish scales, ostracods and plant remains found 

 within its layers and by chemical studies. The color of the rocks is 

 largely due to iron in various states of oxidation, the presence of which 



