178 PEARL SHELDON 
of joints and the continued pressure caused further slipping which 
displaced the joints. It is not necessary to assume that the strike 
and dip joints were formed at exactly the same time. They are 
unlike in other respects and may have been in this. In the experi- 
ments the fine cracks could not be watched as they were formed 
so that their time relation was not determined. 
EVIDENCE OF THE DIKES 
All the known dikes of this region are in the dip joints. None 
are found in the strike ‘set. Kindle’ has suggested that the dip 
joints were older than the Appalachian uplift and the igneous matter 
was intruded before the formation of the strike set. Another sug- 
gestion that has been made assumes that the faces of the strike 
joints were held tightly together by pressure so that the dike 
material could not force its way in. Opposed to this is the fact 
that many of the dike streamers are exceedingly thin, penetrating 
fine breaks. . 
The dikes are mostly in the Portage rocks and in these the dip 
joints are usually the stronger. Judging from the experiments 
the faces of strike joints would be held together but under some 
conditions there might be a tendency for the faces of the dip joints 
to separate. This would allow the igneous matter to enter the dip 
joints more easily. Perhaps the dip joints antedate the strike set, 
even though both were formed during the folding. Since the dikes 
are faulted, they were obviously formed between the time of the 
dip joints and the climax of the faulting. Further study of the 
relations between dikes, joints, and faults would probably be useful 
in determining the exact order of formation of the various sets of 
joints, but from the present evidence it does not seem necessary 
to assume that the dip sets were present before the Appalachian 
Revolution in order to explain the dikes. 
The time of formation of the minor joints is not so evident. 
Probably most of them were formed at about the same time as the 
master joints. Along the walls of the dikes the rocks are cut by 
innumerable small jointlike cracks which were evidently formed 
as a result of the pressure of the dike material.. 
t Folio 169, p. 13; field edition, pp. 96-97. 
