1880.] 419 [Benton. 
prevent, the contact between the igneous and the stratified rocks has 
been found. It is not always easy to find, but when found the line 
of demarkation is very sharp, and the contact phenomena very char- 
acteristic. é 
This line of contact has been observed in at least ten different 
places. In most cases the amygdaloid abuts against the argillite, 
but in some cases against the sandstone, while in one case it is found 
in contact with the conglomerate itself. 
At the Pumping Station quarry, the contact on the north side is a 
very irregular one; the argillite has been baked by the heat of the 
melted amygdaloid matter toa great degree of hardness; its color 
has been changed from gray to a reddish brown; angular fragments 
of it are included in the igneous rock. 
Just west of the Pumping Station, close to the sidewalk on Cam- 
bridge Street, the line of contact is visible for fifty feet, and here the 
plane of contact of the two rocks seems to have an inclination to the 
horizon of at least 15° more than that of the beds of sandstone and 
argillite. The same hardening of the argillite occurs here, and its 
color is altered in a similar manner. 
Under the microscope the line of contact is still more distinctly 
seen than in the quarry or in the hand specimen. ‘The igneous sub- 
stance penetrates between the grains of the fragmental rock, and 
also incloses minute fragments broken off from the latter. 
At the quarry behind the carriage factory on Allston Street, 
occur beds of argillite resting on the amygdaloid, and it is seen that 
they have been tilted up by the pressure of the igneous matter. 
At the head of the lane leading to Allston Heights, and near the 
small powder magazine on Warren Street, it can be seen that the 
amygdaloid has broken through the argillite and sandstone, and here 
the sane hardening and change of color have taken place. 
Under the microscope, small tongues of the igneous matter are . 
seen penetrating the fragmental rock, and the minute elongated feld- 
spar crystals, with which the amygdaloid rock is filled, are seen to 
be arranged parallel to the line of contact. These crystals were 
already formed before the moment at which the rock solidified, and,, 
as would naturally be expected, were forced to take a direction per- 
pendicular to the resisting force. 
A case of the contact of the igneous rock with the conglomerate 
itself has been spoken of ; this occurs on the west side of Howard 
Street, near North Beacon Street, the amygdaloid formation being 
