598 NOTES ON SERPENTINOU3 ROCKS MERRILL. 



felty structure, more like sundry chloritic decomposition products as 

 seen in eruptive rocks than like pure serpentine. 



In the tliiu sections this variety of the rock furnishes no clew what- 

 ever to the origin of the serpentinous material. Sections of the second 

 variety show, however, the rock to have consisted mainly of calcite and 

 dolomite, and that the serpentine is a subsequent injection, replacing 

 wholly or in part the calcite. Sections are readily obtainable showing 

 the calcite granules, with only narrow and irregular veins of the ser- 

 pentinous matter traversing them, through all gradations to complete 

 replacement. It was at first thought that these granules might be 

 dolomitic and actually undergoing alteration into serpentine, but chem- 

 ical and microscopic tests showed them to be nearly pure calcite. The 

 third and fourth varieties mentioned above were likewise found to con- 

 sist of calcite (the coarsely crystalline variety mentioned by Emmons), 

 replaced wholly or in part by the serpentinous matter. Samples were 

 collected, and are now installed in the Museum collections, showing 

 these masses of graphite-bearing calcite in all stages of replacement, 

 from the formation of a ring of serpentinous material around the outer 

 portion (70084) through varieties stained greenish throughout but still 

 effervescing when treated with dilute hydrochloric acid, to compact 

 masses of dark dull green serpentine, still at times showing traces of 

 the calcite cleavage, and carrying as before scales of embedded graph- 

 ite (70083). 



The writer will not attempt to fully explain the source of tbis dark 

 colored aluminous serpentine, which occurs as a true replacement 

 rather than as a metasomatic product. If, as was first supposed, it too 

 was derived from the colorless pyroxene, it is difficult to account for 

 the large increase (6.03 per cent.) of iron oxides and alumina. It seems 

 best to drop the matter here for the present rather than resort to spec- 

 ulations, which may not be borne out by future field observations. 



Thauks are due Mr. S. E. Foote, of Port Henry, but for whose gene- 

 rosity in giving not only his own time, but also furnishing his private 

 conveyance, it would have been impossible in the time at commaud to 

 obtain for the Museum the full set of duplicate material, the collection 

 of which was the main object of my visit. 



B.— AQUEDUCT SHAFT 26, NEW YORK CITY. 



This serpentine occurs in embedded masses in a coarsely crystalline 

 white granular dolomite. It plainly originates through the hydration 

 of a white monoclinic pyroxene, showing under the microscope nearly 

 rectangular prismatic cleavages, and giving extinction angles as high 

 as 44°. The alteration is accompanied with the formation of abundant 

 secondary calcite. The serpentinous matter itself varies from nearly 

 white or colorless to light greenish, or occasionally nearly black ; the 

 green color is never very pronounced. The hardness of the material 

 is a trifle under 4 of Dana's scale, being softer than the Bowenite of 



