44 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES. 
RELATIONS OF JOINTS, DIKES, AND SHEAR ZONES. 
From the similarity of the courses of the diagonal joints and of many of the dikes 
and "hogbacks" in western Vermont and eastern New York there seems to be a 
close relationship between them. 
They may all have been produced by the same stress at the same time, in some 
cases the strain resulting in a hogback, in others in a diagonal joint; and these joints, 
when very deep, may have given rise to dikes. The practical application of this is 
that the possibility of such a relationship should lead the quarryman, whenever he 
finds diagonal jointing, to suspect the proximity of hogbacks and dikes with a similar 
course, and so with either a hogback or a diagonal dike, and this suspicion may 
sometimes save expenditure of time and labor. 
USE OF A GEOLOGICAL MAP AND COMPASS IN PROSPECTING FOR SLATE. 
Although the following paragraphs refer particularly to the western Vermont and 
eastern New York slate belt — and necessarily so because this is the only slate region 
which has been mapped in this way — the general principles and method of procedure 
are applicable to any slate region where the beds consist of close and more or less 
overturned folds. 
The quarry maps, Pis. XX and XXI, are designed to be of practical utility. The 
coloring shows where the Cambrian green and purple and the Ordovician red slates 
may be looked for or not looked for. The dovetailing of the Cambrian and Ordo- 
vician areas represents to a certain extent structural relations and not mere 
"accidents" of erosion. Thus, the Jamesville Cambrian belt is closely related to the 
Cambrian belt which lies east of South Granville. 
On the quarry maps the course of bedding and cleavage has been shown at several 
quarries by special symbols. The scale of these maps is sufficiently large to admit 
the entry of many more quarries and symbols. By using a small geological compass 
to determine the strike of any bed of good slate at any of the located quarries, and 
transferring it to the quarry map by means of a protractor, the probable direction of 
the recurrence of the bed can be ascertained, and so with joints, hogbacks, or dikes. 
Such a compass should be provided with sights, spirit levels, movable ring to set off 
magnetic variation, and have a clinometer attachment to indicate angle of dip. 
Where, as at West Pawlet (see section VII, PI. XXII, and figs. A, B, C, PI. XXIII) , 
the slate is closely folded, a succession of repetitions of the same series of beds may 
be looked for in an east-and-west direction at varying intervals. The possibility of 
the pitching of the axis of a fold in a northerly or southerly direction should be 
looked out for. In such cases older or newer beds are traversed in following the 
direction of the pitch. Where an Ordovician belt abruptly cuts off a Cambrian 
one on the north or south, the Cambrian one must ordinarily be supposed to plunge 
under the Ordovician one. 
From the relations already explained, quarrymen need not be surprised, here and 
there, as the excavation proceeds, to come upon the Ordovician red and bright green 
slates at the bottom of a sea-green or unfading green quarry, or to come upon these 
Cambrian slates at the bottom of a red slate quarry (sections I, II, V, VI, VII, PI. 
XXII). 
Quarrymen are very skilled in detecting the presence of good slate from the peculiar 
appearance of the weathered edge surface, and that skill appears to have been their 
only guide in prospecting in this region. It would be well if this skill were reen- 
forced by the use of the following method in exploration : 
First. Make reference to a geological map for the areas in which the various slates 
may occur. 
Second. Determine on quarry map or general map the good slate beds already 
exploited. 
