1880.) 421 [Benton. 
rence of the amygdules themselves. In another case, the spherical, 
great numbers of amygdules lie close together in spherical clusters. 
Here, also, there is no difference in the groundmass within, and that 
without the clusters. 
In a third case, the annular, the amygdules are grouped so as to 
form concentric rings. Here again, both between the rings, and be- 
tween the separate amygdules, the groundmass is the same as outside. 
In the centre, in many cases, is a cavity lined with minerals. This 
annular arrangement shows well in the quarry at the corner of North 
Beacon Street and Harvard Avenue. 
In the fourth case, the spheroidal, a roughly spherical inclusion 
occurs, which differs from the surrounding mass only by a shade of 
color, and by being crowded with amygdules. The texture of both 
is the same, and they are strictly the same kind of rock. The 
surrounding groundmass penetrates the included fragment in many 
eases. (Harvard and Beacon, also, opp. Carrriage Factory.) 
Fifth case —the angular, is like the preceding, except that the 
‘included fragments are aneular. ‘They are the same kind of rock. 
Thus it will be seen that the different varieties of arrangement of 
the amyedules fall into two very distinct categories. In the /irst 
category, which contains the banded, the ‘spherical and the annular 
arrangements, the groundmass between the amygdules is precisely 
the same as, and strictly continuous with, the great mass of the rock 
No line of demarkation exists between the groundmass within, and 
that without the group. In the second category, which includes the 
spheroidal and angular inclusions, the groundmass ifhmediately sur- 
rounding the amygdules differs from that of the great mass of the 
rock merely by a shade of color. This difference, taken in connec- 
tion with the sharpness of the line of demarkation, and the intrusive 
and penetrative character of the surrounding groundmass, makes it 
very probable that these bodies are included fragments. ‘They are, 
however, fragments of the same kind of rock, their difference in 
color being due to a slight difference in the chemical changes which 
they have undergone since their consolidation. 
Coming now to the groundmass of the amygdaloid in thin sections 
under the microscope, it is found to be composed chiefly of feldspar, 
magnetite, viridite and chlorite. Epidote, hematite, kaolin, and 
other earthy products of decomposition also occur. 
The feldspar occurs throughout the rocks in ledged-shaped crystals, 
with steplike ends. Any two neighboring crystals usually make a 
