280 SMYTH — ROCKS OF NORTHWESTERN ADIRONDACK REGION. 



with low powers these rows look quite like the rutile threads in quartz. 

 Most of the rows are, in basal section, parallel to b. In the symmetry 

 plane the rows are less regular, but usually parallel to c and x. 



The irregular masses of orthoclase which lie imbedded in calcite differ 

 from the foregoing in being semitransparent, with a bluish tinge. Cleav- 

 age fragments indicate that intergrown with the orthoclase there are 

 parallel plates of plagioclase. 



Pyroxene is extremely abundant, and is found both implanted in the 

 fissures of the gabbro and imbedded in the adjacent limestone. In the 

 latter position it sometimes forms individuals two or three inches long 

 and one inch in diameter. The implanted crystals are usually smaller. 

 The color is dark green or black. The faces on the material collected 

 are so imperfect that measurements could not be made, but apparently 

 the crystals are rather simple, probably combinations of b, c, m, n, and o. 



Wollastonite has not been found with crystal form. It occurs in irreg- 

 ular fibrous masses, often several inches in greatest diameter, pure white, 

 and with a pearly luster. 



Scapolite may be in layer-like masses or distinct crystals. The masses 

 are colorless, with a slightly fibrous appearance, on the vitreous cleavage 

 surface. The crystals, which have not been found with a length of more 

 than an inch, are either white, gray or pale blue, 'i'hey show the forms 

 a, m, r and z, some crystals showing clearly the pyramidal hemihedrism. 



Titanite occurs both in the zone of mixed composition and imbedded 

 in pure calcite. The crystals are very perfect, with bright faces and 

 deep reddish brown color. They may be almost microscopic in size or 

 as much as an inch in diameter. They belong to the variety lederite, 

 and show the faces c, m and n. A specimen in the Hamilton College 

 collection from this locality shows the polysynthetic twinning described 

 by Williams* as occurring on titanite from the adjoining town of Pitcairn. 



Zircon occurs in the same manner as the sphene. The crystals are 

 simple, showing only m, p and n. They are often very long and slender. 

 The best specimen obtained was imbedded in calcite; it is about an 

 inch long and doubly terminated. The color in all the specimens in- 

 clines to lavender, which sometimes becomes very marked. 



Other minerals, in particular phlogopite, are present, but as they occur 

 disseminated through the limestone almost everywhere they cannot be 

 regarded as contact-products. In this connection mention should be 

 made of the gieseckite, of which fine specimens are found under and 

 near the natural bridge. The large hexagonal crystals of this mineral com- 

 pare very closely with the form of nepheline, as shown by Blum, who 

 regards the giesecldte as a pseudomorph after the latter species. If this 



* Am. Jour. Sei., third series, vol. xxix, p. 486. ' 



