r24 NEW YORK STATE MUSEUM 



Stark's Knob as a source of road metal. This occurrence of trap 

 affords a small supply of the best road metal obtainable in the 

 upper Hudson valley, and, if judiciously employed as the surface 

 dressing, would serve to make macadamized roads in the 

 vicinity. The outline of the base of the knob has a north and 

 south extension of about 320 feet and a breadth east and west of 

 240 feet. Assuming the volume of trap on this base to have an 

 average thickness of 15 feet above the level at which it might 

 be advantageous to work the material, there are here approxi- 

 mately 117,000 cubic yards of rock in place available for local 

 road metal (see pi. 6). 



PETROGRAPHY AND AGE OF THE NORTHUMBERLAND ROCK 



BY H. P. CUSHING 



Introduction. The material sent me for examination consists 

 of several fragments which have come from either the crusts 

 of the lava balls described by Professor Woodworth, or else 

 from the surrounding ground mass in which the balls are em- 

 bedded; also a single specimen of the rock from the south side 

 of the fault. There are no specimens of the scoriaeeous material 

 forming the inner portion of the balls, nor of the limestone in- 

 clusions. All the material is so badly altered that a satis- 

 factory optical and chemical study of it can not be made, and 

 the rock has no special petrographic interest. The field occur- 

 rence is of great interest, as shown by Professor Woodworth. 

 There remains the matter of correlation. 



Petrographic description. The material from the throat shows 

 a black, fine grained to aphanitic rock, with frequent, calcite- 

 filled amygdules and many seams and patches of the same 

 mineral. The two slides made from it differ mainly in grain. 



The finer grained rock consists of a network of minute feld- 

 spar laths, with a general radial or else spherulitic arrangement. 

 They show no twinning and for the most part extinguish nearly 

 parallel to their longer axes. They are studded with minute 

 magnetite crystals, and belong to the oligoclase andesin series. 



The interspaces are filled with opaque, decomposition prod- 

 ucts. It is not certain that the point of crystallization of augite 



