Mesozoic Igneous rook of New Jersey. 331 



large columns and from different levels of the central mass of 

 small columns show almost no macroscopic differences, but 

 under the microscope a decided variation is noticeable. In 

 most of the thin sections the rock appears but slightly decom- 

 posed ; it is not noncrystalline but contains a variable amount 

 of glass base, which is more or less globulitic with augite mi- 

 crolites having opaque grains attached, besides larger aggrega- 

 tions of magnetite grains. There is a comparatively small 

 amount of light green serpentine in patches, the larger of which 

 still contain fragments of olivine at their centers, the primary 

 mineral from which the serpentine has been derived. In some 

 places the glass base has been colored green though still iso- 

 tropic, while in others it has been devitrified through decom- 

 posing agents. 



The rock with the least glass and coarsest grain of crystalliz- 

 ation is from the large columns about sis feet above the floor 

 of the quarry; that, at the same level but from the central 

 mass of small columns shows nearly the same size of crystals as 

 in the first but more glass base. The rock forming columns a 

 foot thick and fifteen feet above the second specimen has some- 

 what smaller feldspar crystals and more glass base, in places 

 brown and globulitic with fernlike groups of magnetite crys- 

 tals. Midway up the cliff the rock shows still more globulitic 

 and microlitic glass ; and that from ten feet below the present 

 upper surface has smaller crystals and rather more glass base. 



The variations from bottom to top of the lava sheet are slight 

 but distinctly noticeable, and indicate that the cooling which 

 caused the consolidation of the mass was more rapid at the top 

 than at the bottom, which corresponds to the subsequent con- 

 ditions deemed necessary to produce the different systems of 

 columnar cracking. 



This rock is in every way identical with many medium- 

 grained basalts, which have been poured forth as surface flows 

 in recent times, and should be called basalt, and its coarser 

 grained forms dolerite, as Professor E. S. Dana has called simi- 

 lar rocks in the Connecticut Valley. (" Trap Eocks of the 

 Connecticut "Valley," E. S. Dana, Proceedings of the American 

 Association for the Advancement of Science, 23d meeting, 

 1884.) 



The occurrence of the rock in question as a surface flow is 

 rendered highly probable by its glassy nature and the disposi- 

 tion of the columns, which resembles that of many lava sheets 

 in Western America where the irregular cooling may be di- 

 rectly traced to irregularities of surface or to local porosity or 

 cavities within the mass. Another source of irregular cooling 

 may be found in the loss of heat by convection in the atmo- 

 sphere, but more especially in water where the flow has been 

 subaqueous. 



