816 Instruction in Geological Investigation, 
of argument on which geological assertion is based. Praef 
in solving the questions proposed above leads to this desirat 
end. If the dike tilted the slate, the beds should dip away ron l 
the dike on both sides, as in Fig. 4. They do not, and iti 
therefore concluded that the slate was tilted by some force í 
pendent of the dikes. This is soon confirmed by finding that wga 
dip of the slate is maintained at a tolerably even angle for a migi 
Eig t Ryd 
or more, although intersected in various directions by pee 
dikes. If the dike caused the local irregularity in the dip of a 
Slate, the dip should be increased on the south and d a 
e north, as in Fig. 5; but the observed section in Fig. 3% E 
a local increase of dip on both sides of the dike, as if the ge 
had been slightly sheared independently of the intrusion. T 
dikes soon confirm the conclusion that their intrusion i 
perceptibly affect the dip of the adjoining beds. In deciding 
relation between the dates of intrusion and tilting, We ne | 
tule out any argument based on the present tilted attitude ge 
dike ; it may have been vertical originally and then tilted : y 
with the slate to its present dip, or it may have been mi 
its present inclination; therefore no conclusion can be = 
from its dip. The only means here available for reaching 
answer depend on a postulate concerning the joints 1n ee 
The joints may have been made in the slate before “a 
but they were almost surely made while the tilting ieee 
Now, the dike is seen to have smooth joint-faces for the £ 
part of its walls; it must therefore have taken advantag® : 
existent joints in opening a way for its upward escaP® e 
‘its edge would be ragged. The dike therefore — y 
after the joints were made, and probably after * ae 
begun. The want of complete decision in this ¢as° wor 
sidered reason for omitting its consideration. So -n 
logical reasoning leads only to probabilities that the Sti ips 
as well make early acquaintance with this kin 
