OCCLUSION OF IGNEOUS ROCK 409 



elusions. The olivine mesh-structure, as well as the rectangular 

 network of diopside, was distinguished in the thin sections, and 

 even remnants of crystals of both minerals. 



In the Hoboken rock, the fine texture renders the constitu- 

 tion less easily determinable, and from examination of one 

 variety it was described as "a structureless mass of serpentine 

 fibers." Other minerals distinguished were talc, chlorite, dolo- 

 mite, calcite, chromite, with small amounts of iron ores. " No 

 traces of the original silicates could be found in any part of the 

 exposure." An analysis by Goodell was presented, without 

 explanation of the low proportion of silica and remarkable excess 

 of lime, both facts inconsistent with the composition of a " rock 

 thoroughly serpentinized throughout." 



The results will now be presented, from my microscopic ex- 

 amination of 1 3 thin sections, prepared from selected specimens 

 of this Hoboken rock, including light green, dark green, com- 

 pact, laminated and brecciated forms, besides that deposited in 

 veins of marmolite. In even the hand specimens the eye may 

 distinguish a variegation of less altered, dark green spots, and 

 of light green serpentinous spots, and abundantly in both, on 

 certain surfaces, shining green facets, which other observations 

 show to be more or less altered scales of bronzite or bastite. 



In thin sections under the microscope a variety of structures 

 present themselves in great distinctness, particularly the knitted 

 or bar structure, the lattice structure and a columnar structure, 

 though never the mesh structure found in the Staten Island rock. 



In the tracts with knitted structure brilliant veinlets appear, 

 between crossed nicols, in bent or twisted ribbons, greenish to 

 brownish and orange yellow, with decided pleochroism, color- 

 less to yellow and orange yellow ; greatest absorption in direc- 

 tion of the veinlet. Their width rarely exceeds 0.02 millimeter 

 but sometimes reaches 0.04 mm. In the latter case the usual 

 structure is best shown, a central suture or a fine lamination in 

 the plane of the veinlet, with a cross fibration on each side. 

 Occasionally one veinlet is seen to cut across another. The cen- 

 tral plane may exhibit thin continuous sheets, separated by a 

 dark line or suture, and is often nearly isotrope (serpentine). 



