174 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
Tectonic Relations of Joints in the Mystic River Quarries. 
The following changes have, it can he shown, taken place in the 
rocks in the Mystic River quarries : — 
1. The rocks have been very much condensed in volume. This 
is shown by their containing less water than in their original sedi¬ 
mentary condition, and by their present compact structure. 
2. The strata have been tilted to an angle of about 25 degrees 
of inclination, but not equally, so that some degree of torsion must 
have acted upon them. 
3. The rocks have been invaded at least twice by igneous rocks 
at high temperatures, in the form of dikes and sills. Both country 
rock and intrusives long since cooled down. The intrusive rocks 
show abundant signs of their contraction. Similar effects might be 
expected in the country rock. 
These three changes have been supposed to account for certain 
joints, and experiments have been made to demonstrate the com¬ 
petency of the respective agents in particular cases. 
In the Mystic River quarries the feather-fracture planes are well 
developed as “dip” joints affecting single layers. Parallel with 
them are master joints occupied by diabase dikes. These dikes as 
pointed out by Davis, are newer than the joints they occupy and are, 
therefore, not causally related to them. While the joints limited to 
a single bed are, as above noted, parallel with these master joints, it 
does not follow that they are of the same age. The absence of 
phenomena of intersection with these joints renders difficult the 
working out, on the ground, of a causal relation to the dikes of the 
later invasion. I think the joints are due to contraction. 
Detection of jo int-planes and master joints : — There are occa¬ 
sions when it is desirable to ascertain the presence of fissures or 
joints in underlying strata. It follows from the relation of the 
oblique b-planes in the fringe of a joint, that where on a horizontal 
surface, for instance, these oblique fractures occur with the appearance 
shown on Plate 1, fig. 8, the presence of a joint having the general 
direction of the zone of small fractures is to be suspected. Exami¬ 
nation of the surface of one of the small b-planes should be made, 
where possible, to determine the presence of feather-fracture. If 
this feather-fracture exhibits a convergence of the feathery lines 
downward, the master joint or joint-plane proper will be met with in 
the subjacent rock ; if the feather-fracture converges upward, then 
