1880.] 360 [Diller. 
is true of Cliftondale where the dikes of felsite are more distinctly 
marked than on Marblehead Neck. 
The breccia of the Malden Highlands is composed almost wholly 
of fragments of silicious felsites. The “basic felsite” completely 
surrounds the brecciated area furthest north along the Malden- 
Medford line, and yet there are very few or no pebbles of “basic 
felsite” in this breccia. The facts indicate that the breccia was 
formed before the “ basic felsite”’ occupied its present position. 
The base of Prospect Hill, Malden, is breccia, and its top is 
‘basic felsite.’’? The same is true of the hill a short distance north- 
east of Oak Grove station; and in a number of other places the 
“basic felsite” seems to overlie the breccia. ‘This evidence is in 
harmony with that already noticed, and makes it probable that the 
breccia is older than the “ basic felsite.” 
It has already been stated that the line of contact between the 
felsites and the breccia is sometimes quite difficult to determine, but 
the reason for this difficulty will be readily comprehended when we 
understand the genetic relations of the two rocks. The breccia 
occurs at intervals throughout the whole area occupied by the 
felsites. Just before the more recent eruptions of felsite, the brec- 
ciated materials were scattered over the surface, and the fragments 
may have been loose, or only partially cemented together. The 
felsitic lava, at the time of its eruption, flowing over the surface, 
picked up many loose fragments and became very intimately and 
irregularly mixed with the materials of the breccia. 
There was thus produced along the line of contact between the 
felsite and the breccia, a felsite containing many fragments cemented 
together by a paste of igneous origin. The number of fragments 
decreases with the increase of distance from the line of contact, and 
the felsite containing the included fragments, may be said to pass, 
on the one side, by a gradual transition into compact felsite. There 
is however, no such passage in the opposite direction, 7. e., between 
the felsite with included fragments and breccia. There is always a 
line of contact between these rocks in nature, but it is sometimes 
quite difficult to discover. As might be expected, an observer in 
the field may be in doubt whether a certain rock is breccia or felsite, 
and it may cost him much labor to find the lines of contact and 
settle the question. In this case the microscope becomes a very 
valuable help, for by using it to study a thin section of the rock 
the observer can readily tell whether the rock is eruptive or sedi- 
