- 1887] | Instruction in Geological Investigation. 817 
_ demonstration, but great care must be taken'to see that he fully 
= Tecognizes the postulates of the argument and does not over- 
_ draw on their certainty. The desire for final settlement of all 
_ Questions is strong in the student mind; questions that must 
q be held open for further investigation are not always favorites, 
but I believe they are aids to the habit of mental deliberation. 
In this particular case the doubt is soon lessened, for a little 
kter, on the same excursion, a ragged dike is found breaking its 
_ Way unevenly through the slate. The slate is jointed now, and 
_ traversed by smooth-walled dikes, showing that the irregularity 
_ of the ragged dike is due not to the impossibility of smooth 
p” being made in the country-rock, but to their absence at 
_ the time of its intrusion. Finally, a smooth dike is found inter- 
p secting a.ragged dike, giving new and safe evidence of the rela- 
p medy early and late dates of their respective intrusions; and 
i this is.emphasized as a good illustration of a double approach to 
 4single conclusion. ; 
One of the m 
ost important exercises in connection with the 
work is the careful formulation, oral and written, of the 
_ Conclusions reached and the means of reaching them. It helps 
4 to do away with the idea that a geologist is a collector of speci- 
_ ‘Mns rather than 4 collector of facts and arguments; it is good 
4 mental practice, and aids the student in any kind of work he 
d T afterwards undertake ; It is to most of the class the greatest 
Profit that they carry from their excursions. 
; | of the most instructive quarries in Somerville is roughly 
a wn in Fig. 6. The attempt is here made to show the visible 
é and Covered ro 
. 
aoe ck areas, but it should be remembered that the 
q Visible Tock is 
much less distinct in the dusty quarry than in the 
dike m. The tilted slates, S, S, are intersected by a forty-foot 
“thigh wD: and both are cut by a double dike, CD, of moderate 
Poe The large dike is soon seen to be horizontally faulted, 
io. is put, When and where was it dislocated? the 
Wer generally refers the time and place to the newer dike, 
of slate Opposite the slate wall, and of old-dike rock 
Without the old dike. The newer dike therefore opened its way 
_ any lateral shift of its walls. This is confirmed by 
