VEINS NODULES DIKES. 33 
segregation. In some cases there is a rough parallelism in them. They may occur 
partly in the bedding and partly in the cleavage, or may cross both bedding and 
cleavage, their general dip being roughly at right angles to that of the cleavage, and 
their strike probably not very different from that of cleavage and bedding. Such 
veins may be several hundred feet long and several feet thick, or may form lenses 
several feet in length. At Benson, Vt., and Ijamsville, Md., small joint veins of 
fibrous calcite form series of gashes "en echelon." The fibers or prisms lie trans- 
verse to the course of the vein. Daubree regards such veins as due to stretching." 
That the material of the veins described, of whatever sort, must have come from the 
adjacent slate itself is evident, and also that it was deposited in solution. 
In the cases adduced there is some relation between the foliations of the slate and 
the course of the veins, but veins do occur which appear to be quite lawless, crossing 
in every direction, anastomosing, and intersecting one another, and sometimes 
inclosing fragments of the adjacent slate and constituting the cement of a brecciated 
area. That the veins are the result of various secondary stresses, producing openings 
of more or less irregularity, is manifest. Where the stress ceased to operate the vein 
tapers out; where the stresses were complex the veins are so. 
The material which filled the openings thus made is chiefly quartz, frequently 
"milky" in color, and often finely crystallized in small cavities. With the quartz 
are often associated chlorite, biotite, calcite, and possibly dolomite. The chlorite 
sometimes occurs in hexagonal scales, in vermicular aggregations, or in tortuous 
columns. Some of the smaller veins are banded, presenting alternations of quartz 
and fibrous calcite, or of quartz and rhombs of calcite, as in the case of those in plicated 
joints described on page 28. Sometimes the quartz contains cavities measuring from 
0.002 to 0.005 millimeter in diameter, partly filled with fluid. Galenite in small 
particles was found in veins at the Jamesville, N. Y., quarries. 
NODULES. 
In some of the western Vermont quarries very hard nodules, a few inches in diam- 
eter and of lenticular form, occur along the bedding planes. They consist of a 
quartzite nucleus containing much calcite and large scales of muscovite surrounded 
by slaty layers with calcite, quartz, and muscovite scales. Pyrite is disseminated 
throughout both nucleus and outer zone. Such nodules are evidently of sedimentary 
origin. 
Miiller describes concretions of pyrite and quartz in the slate quarries of Thuringen 
and observes that the slate in their vicinity is of superior quality, containing less 
pyrite than it does at a distance from the nodules. The explanation is that all the 
pyrite has congregated in the large nodules instead of being more widely disseminated 
in small crystals. The nodules are thus of economic advantage. & 
DIKES. 
Volcanic dikes are not infrequent in slate deposits. Leaving out the very excep- 
tional one described by Mr. Eckel on page 57, dikes are almost as detrimental as 
veins. They are usually more regular than veins of segregation and, as stated under 
"Joints," are apt to give warning of their proximity by an abundance of joints par- 
allel to them. For several feet on each side of the dike these joints become very 
close and are sometimes crossed by an equally numerous set of horizontal ones, thus 
breaking up the slate into small blocks. At Arvonia, Va., it appears that the quality 
of the slate a little beyond the dike is better than it is farther away, so that a dike 
traversing a slate deposit is not necessarily without economic value. 
« Daubree, Geologie experimentale, p. 144, fig. 166. 
bMviller, F. E., Die normalen Sehiefer des Hennbergs,p.218. (See Bibliography, pp. 138-145.) 
