40 THE GRANITES OF MAINE. 
such a mass is left as the head or wall of the quarry. PI, VIII, A, 
shows a typical heading on Dix Island. PI. IX. />. from a pho- 
tograph taken at the Longfellow quarry, near Hallowell, shows the 
intersection of two headings, one striking about northeast, the other 
about northwest; also the flexuous course of the northwest set of 
joints. Headings afford ample ingress for surface water, and con- 
sequently the granite within a heading is generally badly stained, if 
not decomposed. This will be referred to more fully under the 
heading " Decomposition " (p. 54). 
An interesting feature of both headings and joints shown in some 
of the deeper quarries at Quincy, Mass., which may be found in 
Maine as the quarries are deepened, is their vertical discontinuity. 
A heading occurring at the surface may disappear below, or a head- 
ing may abruptly appear a hundred Peel below the surface and con- 
tinue downward. 
Headings are not easily accounted for. They may he produced by 
vibratory strains that recur at intervals of time. If they arc so 
caused, the character of the fractures in some headings indicate that 
the strains arc very complex. 
The courses of headings at each quarry are given in the descrip- 
tions of the quarries in Part 1 1. 
The polished aiul grooved faces (" slickensides ") observed on many 
of the joint- at the quarries -how that faulting has occurred along 
them. The discontinuity of the sheets at some of the joint-, causing, 
where the joints are slightly inclined, what quarrymen call "toeing 
in," may probably he attributed to faulting. This supposition as- 
sumes, of course, that the sheet structure was formed prior to the 
jointing. There seems to he good evidence of- faulting on a consider- 
able scale along the joints at Dodlin Hill, near Norridgewock, the 
details of which are described on page L50 and illustrated in fig. 33. 
Faulting occurs also along sheet-, displacing vertical flow struc- 
ture, at the same quarry ( p. 25), as well as displacing vertical dikes, 
as at the Allen quarry, on Mount Desert, as shown in PI. VIII, //. 
and referred to on page 100. The lateral faulting here has occurred 
both in northeast-southwest and in east-west directions. Another 
faulted dike is mentioned on page 110. 
\\ [CROSCOPN FBACTUEES. 
In some of tin 1 Maine quarries the granite near the surface acquires 
a marked foliation, which appears to he parallel to the sheet struc- 
ture, and possibly to the rift. This foliation is known by quarry- 
men as "shakes." It occures both at the top and at the bottom of 
