J. V. Lewis — Palisade Diabase of New Jersey. 157 



areas are sometimes as much as 3 or 4 millimeters across, and 

 are then distinctly visible in the hand specimen, as in the 

 western portion of the Pennsylvania railroad tunnels at Home- 

 stead. Frequently individual grains of quartz, and less com- 

 monly of orthoclase, are also observed, attaining in some 

 instances a diameter of 1 millimeter. 



Magnetite is always present but in greatly varying amount. 

 Crystals are sometimes observed, but most of it, like the augite, 

 is irregularly clustered between the plagioclases, and some- 

 times partly incloses both the plagioclase and the augite. The 

 frequent presence of magnetite secondary from the alter- 

 ation of augite renders it impossible in many cases to distin- 

 guish with certainty that of primary origin. It is probable 

 that masses molded about the other constituents are largely 

 composed of secondary accretions. 



Eiotite is also often present in small amount, and is usually 

 clustered about the magnetite in relatively large irregular 

 flakes. It is strongly pleochroic — deep reddish brown and 

 light yellow. Some secondary biotite, after augite, occurs, 

 but in most cases this is readily distinguished from the primary 

 mineral. 



Olivine is absent from the great bulk of the rock. It occurs 

 in small amounts, however, near the contacts with the inclos- 

 ing strata, and is exceptionally abundant in the oli vine-diabase 

 ledge of the Palisades, constituting as much as one-fifth of the 

 whole. In the line-grained border facies of the rock it occurs 

 in scattering porphyritic crystals, which sometimes exhibit 

 resorption phenomena in rounded and embayed outlines. 

 Corrosion mantles or "reaction rims" of radial enstatite 

 occasionally surround the larger crystals, and nest-like aggre- 

 gates of it entirely replace some of the smaller ones. In this 

 part of the rock mass the olivine is usually more or less altered 

 into yellowish or brownish serpentine ; but in the olivine-diabase 

 ledge it occurs in numerous perfectly fresh crystals and irregu- 

 lar grains. Most of it forms poikilitic inclusions in the feld- 

 spars and less abundantly in the augites, and it retains a 

 striking freshness and transparency even in the presence of 

 considerable alteration of the augite. 



Apatite is always in well-formed prismatic crystals, ranging 

 from very minute up to 1 millimeter in length and 0*06 milli- 

 meter in diameter. It is always abundant in the feldspars and 

 quartz, sometimes in plagioclase, sometimes in the quartz- 

 orthoclase intergrowth, and is rarely seen in the other consti- 

 tuents. 



Chemical composition. — From a number of analyses that 

 have been made the following are selected to show the range 

 of composition of typical facies of the rock : 



