duaia an 
` dimensions of length 
 Pends; and this leads to 
* y 3 
Did the 
; ? 
: befo 
Tk Noth 
 Ceiye that he 
1887] Instruction in Geological Investigation. 815 
parts of the crevices thus opened shows how mobile it was under 
the heat and pressure that caused its motion. This specific detail 
carries the observer back to the time of the intrusion, and gives 
reality to it, 
A cross-section of the quarry is then asked for, and this brings 
up a very common difficulty arising from lack of practical geomet- 
rical knowledge. The era 
same difficulty appears 4 LIAR NY 
in determining the di ` 
a dike; Gee R 
V 
the rocks were to be EG \ 
studied only in the two N 
gow 
and breadth, without NWL 
the third dimension of N 
thickness or depth. LUG g, 
Even so simple a geo- 
metrical matter as strike has generally to be labored over. The 
*0ss-section of the dike and slate is drawn to estimated scale 
with some care (Fig. 3) 
ect under-ground inter- 
Pretation, on which so 
much advanced work de- 
ew questions: 
dike tilt the 
id the dike : 
Variation in the dip of the slate? Did the dike appear 
re or after the slate was tilted ? 
Such 
figs 
Cause the 
! abl questions as these appear utterly obscure and unanswer- 
€ to 
Many students: not because they have had no practice 
x ological investigation, but because they are unpractised n 
vestigation itself. The relation of observation, hypothesis, an 
_ ol has not been unfolded to their minds, and it is for this 
i abst | lay so much stress both here and in the field on the 
means of reasoning, as well as of observation, in geological 
ing is more important than for the student to per- 
May discover and follow the legitimate sequence 
