VEINLETS IN THE SILURIAN AND DEVONIAN 63 
great as 2 mm., and the fibrous structure is hardly noticeable in the 
narrower veins. Where veins of fibrous calcite in the Onondaga 
“blue limestone” pass through chert nodules, there is a sharp 
change in texture, the veins becoming coarsely crystalline and non- 
fibrous within the chert. This sudden change in texture is easily 
noticeable in veins that are less than a millimeter in width, when 
they are examined in thin sections under the microscope. 
Where the veins are non-fibrous, the individual crystals usually 
have their longer dimensional axes parallel rather than transverse 
to the strike of the veins; and, especially in the smaller veins, most 
of the crystals extend from wall to wall. In the larger veins these 
crystals have maximum diameters of over 5 cm., while in the fibrous 
portion of the same veins the fiber crystals are uniformly 0.1 mm. 
or less in diameter. The larger crystals of calcite frequently show 
warped cleavages, and under the microscope undulatory extinction 
is common in these crystals and also in those of fibrous form. The 
fibrous crystals are very irregular in cross-section, since the prisms 
are not bounded by plane surfaces as is often true of the crystals 
found in the non-fibrous portions of the veins. 
Vugs lined with calcite crystals of normal habit (simple rhom- 
bohedrons with some scalenohedrons) are occasionally present in 
the fibrous portion of the veins where the walls are of limestone, but 
they are more abundant where the veins are coarsely crystalline 
and have chert walls. The walls of the veins are sharply defined, 
and inclusions of the wall rock, limestone as well as chert, are com- 
mon. When one wall of a vein contains angles or other irregular- 
ities, there are corresponding irregularities in the opposite wall, 
such that the two surfaces would fit closely together if placed in 
contact. 
ORIGIN OF THE FIBROUS VEINS 
In previous papers’ the writer has cited evidence tending to 
prove that cross-fiber veins of the asbestiform minerals could not 
have been formed through any process of replacement or of recrys- 
tallization 7m situ and that they were not deposited in open fissures. 
«Stephen Taber, ‘“‘The Origin of Veins of the Asbestiform Minerals,” Proc. Nat. 
Acad. Sci., II (1916), 659-64; and ‘‘The Genesis of Asbestos and Asbestiform 
Minerals,” Bull. Am. Inst. Min. Eng. No. 119, 1916, pp. 1973-08. 
