HAWKINS, LOCKATONG FORMATION OF THE TRIASSIC 155 



Petrography and Chemistry 



Under the microscope, little can be determined regarding these rocks, 

 except by the use of high-power objectives. The most typical argillite of 

 the densest type appears to be composed mainly of tiny quartz grains of 

 an extremely angular shape, together with a few bleached biotites, and, 

 at times, a little feldspar. In some cases, the feldspars become much 

 more numerous ; some of them show well developed plagioclase twinning 

 in wide bands, although the majority are orthoclase. The feldspars are 

 angular cleavage fragments, and appear for the most part perfectly fresh. 

 They are evidently primary constituents, as also the biotite appears to be. 

 The biotite is in small lath-shaped fragments, pale brown in color, show- 

 ing traces of parallel cleavage and high order interference colors, high 

 relief and parallel extinction. The whole aggregate resembles closely an 

 assemblage of the constituents of some of the gneisses from whose disin- 

 tegration products the Triassic rocks were undoubtedly derived. 



At Byram, Hunterdon Co., N". J., there is a large active quarry in the 

 dense massive layers of the Lockatong. At this locality, as Professor 

 J. V. Lewis^* has pointed out, the rock is much indurated on account of 

 the proximity of a diabase sill of considerable size and extent. Micro- 

 scopic investigation of the quarry rock shows that its ground-mass is 

 thoroughly re-crystallized, giving to it an almost flinty hardness, so that 

 it rings when struck with the hammer. The sediments close to the dike 

 are re-crystallized also, and in them there are biotites and hornblendes 

 which seem to have been produced in situ, with some areas which appear 

 to be scapolite, as originally described in the publication above noted. 

 The rock at this place is a true hornfels. The effects of the diabase, how- 

 ever, die out within a few hundred feet of it, and such biotites and feld- 

 spars as are found in the rocks far removed from igneous action appear 

 to be purely clastic. 



The Lockatong series is for the most part free from visible igneous 

 rocks, though it is to be noted that a number of narrow dikes appear. 

 The Lockatong strata in general do not strongly resemble those which 

 have been "baked^^ by the intrusives, although the derivation of the silica 

 cement, described below, from hot solutions emanating from the intru- 

 sives does not seem impossible. The color of the various strata, however, 

 is easily accounted for, with the help of the chemical data at hand, with- 

 out appealing to igneous action. The reddish and gray phases of the 

 massive argillites are very much alike in eYery way except color. A 

 massive gray layer is often seen to change upward or downward to a red- 



"Ann. Kept. State Geol. N. J.. 1908. p. 05. 



