174 The American Geologist. b-^P-ember. 1901. 
bv extensive dikes of granite and pej:^niatyte which have a 
northeast-southwest direction and have no apparent connection 
with the granite mass to the south. Near the eastern end of 
the main belt there is a Triassic diabase dike which has been 
traced for a ch stance of seventy miles through southeastern 
Pennsylvania and Maryland. 
Where the granites border the area of basic eruptives on the 
south there is a gradual passage of the granite into dioryte and 
no line of separation can be drawn between them. The noryte 
forms the bulk of the main mass of basic rocks and the hyper- 
sthene-gabbro is to be regarded as a facies of the noryte. The 
latter rock is in some places found to graduate by intermediate 
tvpes into pyroxenyte and the two occur intimately associated 
in the field. The pyroxenytes and peridotytes are closely re- 
lated and they commonly occur together, in one instance both 
being found in the same dike. 
A study of their geological relations and occurrence makes 
it evident that the rocks of this area were not all formed at the 
same time. It is probable that the region was the scene of 
eruptive activity for a considerable period, during which the 
different types were produced. The norytes and gabbros ap- 
pear to have been the first to be erupted. The diorytes were 
r>robably formed at the same or nearly the same time. 
The absence, in Cecil county, of intermediate types between the 
pyroxene and hornblende rocks may indicate that they are of 
different ages, and it is not unlikely that the formation of the 
dioryte followed shortly after that of the norytes and gabbros. 
Together these three types compose the long narrow belt of 
basic rocks that have broken through the ancient gneisses. 
It is difficult to give with any degree of certainty the rela- 
tive age of the granites since the requisite data for the de- 
termination of this are wanting. It seems probable, however, 
that they are younger than the norytes and diorytes. The 
presence of the numerous granite and pegmatyte dikes in the 
basic rocks may indicate that the granites of the main mass 
lying to the south are younger, though no connection between 
these dikes and the larger granite area was observed. The 
gradation of the granite into dioryte and the absence of any 
line of separation between them is thought to be evidence that 
their difference of age is not very great. 
