HOW SCENERY IS MADE 
•15 
The primary cause appears to be local sHrinkings of the 
crust of the earth. It is well known that temperature rises 
about 1° for every 100 feet of descent; so that at some un- 
known depth all substances of the earth must be molten, unless 
pressure or other influences intervene to alter the above rate of 
increase of temperature. As the earth parts with its heat by 
radiation, and has done so to a far greater extent in former 
times than now, the superficial layers must contract, and so 
get crumpled up to adjust themselves upon the underlying 
rocks. 
Splendid illustrations of such occur, like a series of waves, 
along the coast of Cornwall and Pembrokeshire ; and may often 
be seen on the lofty peaks of Switzerland, &c. The generally 
parallel lines of mountain ranges follow roughly these waves in 
the rocks. The heat produced by contraction — the force being 
converted into heat, just as when a gimlet is bored into hard 
wood it gets hot — finds vent in outbursts of molten matter aided 
by the agency of expansive gases and steam. Volcanoes are 
the result. As almost all active volcanoes are situated near the 
coast, and as Vesuvius, for example, when quiet, still pours out 
water vapour, it is reasonable to suppose that the sea has access 
through cracks to the subterranean regions and so generates 
steam, which is thus probably the main cause of explosions. If 
the disturbance be, comparatively speaking, superficial, small 
Puys, which soon get exhausted, are the result. But more 
deeply-seated and more permanent disturbances give rise to 
permanent volcanoes, forming sometimes gigantic cones, as in 
the Andes. 
The relative ages of volcanic disturbances through past 
geological history can be pretty well determined by means of 
the sedimentary rocks through which they have issued. Thus, 
while the Malvern hills are some of the oldest in the world, the 
Alps and Himalayas are almost, so to say, of yesterday’s creation. 
The value of lava and deeper-seated igneous material is very 
great. Thus the former is now seen as basalt, one of the best 
and hardest of materials for road metal ; while the deep-seated 
granite is invaluable for structural work where ornamental 
pillars, &c., are required, to which a high polish can be imparted. 
Great oblong crystals of felspar, red or white, are often seen in 
granite. These were formed as it cooled. 
Granite, though so hard, readily decomposes on exposure. 
Its main ingredients are quartz, felspar and mica; the felspar 
decomposing into powder is called kaolin, and when washed 
clear of other ingredients it is invaluable for making china, 
porcelain, pottery, &c. 
Though diamonds are usually obtained from alluvdal wash- 
ings, those from Kimberley are extracted from the neck of a 
volcano ; but the conditions under which they, like plumbago, 
were made from vegetable matter are unknown. 
George Henslow. 
