HAWKINS, LOCKATONO FORMATION OF THE TRIASSIC 157 



Certain limestone layers that are occasionally developed to a thickness 

 of one or two inches, which appear light gray and coarsely crystalline on 

 the fresh fracture, but which rust on weathering, contain, on the aver- 

 age, nine per cent of MgO. They are therefore not true dolomites, but 

 should be called magnesian limestones. 



The color of all the blackest shales of this formation is due to carbon, 

 which burns off from the powdered sample, heated in a crucible nearly to 

 redness, usually leaving the sample gray in color. The origin of the 

 carbon is in the organic remains, of which traces can usually be found 

 somewhere in such beds. Part of the color of the massive gray layers is 

 due to the same cause. 



The iron compound in the argillite is not the cementing material, 

 since, if boiled in concentrated hydrochloric acid for some time, the rocks 

 lose the color from their surface, but the interior of the mass does not 

 disintegrate in the smallest degree. Under similar treatment both the 

 red and the gray rocks of the type referred to behave similarly. 



The cement of the strongest and most compact of the massive argil- 

 lites (those described above, for instance) is opaline silica. The slow 

 maceration. of a small solid sample of the rock in a concentrated solution 

 of sodium hydroxide on the water bath, for from 36 to 48 hours, reduces 

 a considerable part of it to slimy mud.^^ The opaline cement was ob- 

 served some years ago in sections of the rocks by Professor J. Y. Lewis. ^^ 

 The rocks of this series are, however, of a widely varied character, some 

 layers showing a more calcareous cement, and others having very thin 

 limy layers intercalated with siliceous ones. 



Some of the feldspar fragments observed in this rock under the micro- 

 scope have cloudy or kaolinized borders, and doubtless much finer feld- 

 spathic material originally present has disappeared in this way. Kaolin- 

 ization of feldspars always sets free silica (Clarke). This silica is in a 

 condition to be readily taken into solution by waters of any kind. In this 

 dissolved condition the silica may be imagined to have existed in the still 

 moist muds of the Lockatong. From these solutions the silica would then 

 be deposited as the muds dried. Thus its introduction might be sup- 

 posed to have taken place contemporaneously with the deposition of the 

 sediments. If, on the other hand, the cementation occurred in the sedi- 

 ments sooner or later after their deposition, the silica might have been 

 introduced at a late period, as some observers have believed, in connection 

 with the great outburst of igneous activity which marked' the latter part 



15 Spring^ W. Uebei- die eisenhaltigen Farbstoffe sedimentarer Erdboden und iiber 

 den wahrscheinlichen Urspruug der rothen Felsen. Neues Jabrb. fiir Min.. Geol. u. 

 Paleont. Jahrg., 1899. p. 47. 



18 Ann, Kept. State Geol. N. J., 1908. p. 95. 



