/ £22  GEOGNOSY AND GEOLOGY. 
inclined at various intermediate angles. The horizontal position is of 
least interest, what little it possesses arising from its relation to the inclined. 
In inclined strata the geognosist has first to deal with dip or inclination 
and strike. The dip of a stratum is the angle which it makes with a 
horizontal plane, and the strike the angle made by a horizontal line of the 
stratum with the meridian. The direction has also to be considered in the 
dip. For determinations of dip and strike, the mining compass, with a 
pendulum and graduated arc attached, is the most convenient instrument. 
The planes of stratification are either parallel or convergent to a greater 
or less degree. When the latter is the case a fan-shaped stratification is 
presented, in which the planes of stratification all appear to converge 
towards one point, diverging from one another in the upposite direction. 
9. Relation of Stratification to Mountain Masses. 
The relation of stratification to mountain masses is of great importance, 
their whole character depending on it. Mountains and valleys are arranged 
similarly with their predominant strata. It has already been mentioned that 
stratification is peculiar to rocks which have been deposited from water ; it 
will therefore be readily understood that the general arrangement of strata 
must be horizontal, or not very far from this position. That all strata, 
however inclined, contorted, broken, or disturbed, were really once hori- 
zontal, is a proposition which admits of no doubt, with the powerful reasons 
in its favor furnished by geological science. The question immediately 
presents itself, however, by what means have the strata been elevated ? 
What kind of force has produced such effects? The answer to these - 
queries we find in the investigation of those rocks already ascertained to be 
of igneous origin. The peculiar manner in which these latter occur, the 
relation in which they stand to the stratified masses, and the alterations they 
effect in the petrographical condition of the same, fairly authorize us to look 
upon such igneous masses as closely connected with the phenomena in 
question. Thus, on the coast of Dorsetshire we shall find beds of chalk 
upheaved by basalt ( pl. 52, fig. 8). An upheaval of rocks of the Jura 
( pl. 43, fig. 13, b, c, d) by abnormal masses is seen near Freiberg in 
Breisnau, and in the canton Bern (fig. 14). This will be referred to more 
fully hereafter ; at present our main object is to consider the relation of 
strata to mountain masses. To do this properly it would, perhaps, be 
convenient to name valleys according to their origin. The difficulty here, 
however, would be in the introduction of theoretical views into nomenclature, 
which might be embarrassing to a beginner in science. Thus, longitudinal 
valleys might be called valleys of elevation, their formation being contem- 
poraneous with the elevation of the mountains. A longitudinal valley is 
generally so constituted that its strata are parallel to the slope of the 
mountain which has given rise to it; thus, if we suppose horizontal strata to 
be elevated by two forces acting parallel to each other, the valley will lie 
hetween the two mountain ridges thus produced. Single circular valleys 
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