N. N. 
1/ 
ECONOMIC GEOLOGY OF SLATE. 43 
moderately large area were fewer than between those brought in at different time* at 
any one spot. Cleavage being the result of a later compression may traverse sediments 
of slightly different composition with ltttle change indirection, but will be very much 
affected by great changes in the material or the grain of the sediment (see PI. II, B). 
The prime factor is, then, the bed, the second one the cleavage. 
In the southern part of the slate belt of western Vermont, where beds of quartzite 
or limestone are few and inconspicuous and the difference of color is slight, the dis- 
tinction between bedding and cleavage is not easily made. Quarrymen and pros- 
pectors sometimes regard them as identical when they differ. Where the strikes of 
the bedding and cleavage are divergent, if that of the cleavage be mistaken for that 
of the bedding, a new opening may easily be made at 
the wrong point and the looked-for bed may be missed 
(see fig. 3). In such places the readiest means of dis- 
tinguishing cleavage and bedding are: 
(1) The fossil impressions (trails or algae, sometimes 
called "wavers") are always on a bed surface. 
(2) Minute plicated beds of calcite and quartz indi- ^ £ 
cate bedding. «5? "° 
(3) A microscopic section transverse to the cleavage, 
if other means fail, may indicate the amount of diver- 
gence between the bedding and the cleavage (see fig. 
c, pi. vi). n 
In some places, however, bedding and cleavage are I " ' 
identical in both strike and dip. 
" FLINTS "—THEIR NATURE AND CAUSE. p 
Beds of quartzite, often calcareous, micaceous, pyri- , c/eav.symt>o/ 
tiferous (see PI. V, .4), should never be confounded symbo/ A •— 
with veins of quartz (see PI. V, B). In some regions / 
they are both indiscriminately designated by the FlG . 3 ._ D iagram illustrating the 
quarrymen as "flints." The former are sediments effect of divergence in strike 
mainly of quartz sand, and, although varying consid- of slate bedding and cleavage, 
erably in thickness, are generally more persistent than 
the veins which, as has been already shown, are chemical infiltrations into fractures 
produced at a much later time in consequence of various stresses. Ordinarily the 
quartzite has a more granular and less glassy surface than the vein quartz. A micro- 
scopic section under polarized light will almost always show the difference when 
ordinary means fail. The importance of not confounding the quartzite beds and 
the quartz veins lies in this — that while the quartzite beds indicate the direction and 
thickness of adjacent beds of slate, and thus prove helpful, the quartz veins consti- 
tute perhaps the most fortuitous and the most pernicious element in slate quarrying. 
The strains which the slate masses have suffered have been so various that it is almost 
impossible to foretell the probable presence, course, extent, or thickness of a quartz 
vein. A few things should, however, be noted. While the fractures which occa- 
sioned the veins are to be looked upon somewhat as accidental, they are the result of 
stresses affecting large areas or of the complex interactions of pressure in a few defi- 
nite directions. The course of a vein which is tapering out should be taken with a 
compass, and another should be somewhat expected in the same line or in directions 
parallel to it, or at nearly a right angle to it. 
Bull. 275—06 4 
