26 THE OPEN BOOK OF NATURE 
study window at the present moment. I look upon 
the Renfrewshire Hills, with their greatest height, 
Misty Law (1,663 feet) in the distance. These hills 
are all the result of volcanic activity in a past 
geological epoch ; they are built up of a succession of 
lava-flows from some ancient vents. 
Volcanic rocks also occuras “‘ dykes” and “sills.” 
Dykes have been formed by molten stuff rising from 
¥ ocean nl Si 7) 
ee aa Pail Mi fil 
TM Ss. id yy), 
, Seg ag ee Ly 
Ol fp sf Re < ii BEE 
‘ = eth us Mike 
ANNA 7 ”, Y 
( HUE ie tise 
SEM Me H ZS 
a Y/ 
ay lie iy ei 7' 3 
a : 
or Bh ath 
fe RA ; 
Bb \S + B 
= == =o x f 
eS  —— Se = — a 
—— el z . ie $ 
aoe a @ 
ee é = = 
(Naren Sas == = é uy 
\ x To eee ay 
SS See SS ee Se e | Sad 
S . : z 
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us J 
Fig. 1.—SKETCH MADE FROM PHOTOGRAPH OF QUARRY AT POUK 
Hitt, STAFFORDSHIRE, SHOWING IGNEOUS Rock (Basatt), 
WHICH IN COOLING ASSUMED A COLUMNAR STRUCTURE. 
the deeps of the earth in times of volcanic activity, 
and filling fissures in the crust. You may observe 
dykes of volcanic origin filling fissures in rocks that 
were formed in water. Sometimes the dykes stand 
out like long walls of various thicknesses, and their 
outlines and direction can be traced for miles. 
Softer sandstones in the fissures of which dykes have 
been formed often wear away, or “ weather,” as 
