MOUNTAIN SLIDES. 289 
In nearly every case of this sort the circumstances of which 
are known—except the rare instances attributable to earth- 
quakes—the immediate cause of the slip has been the imbibi- 
tion of water in large quantities by bare earth, or its introdue- 
tion between or beneath solid strata. If water insinuates itself 
between the strata, it creates a sliding surface, or it may, by 
its expansion in freezing, separate beds of rock, which had 
been nearly continuous before, widely enough to allow the 
gravitation of the superincumbent mass to overcome the re- 
sistance afforded by inequalities of face and by friction; if it 
finds its way beneath hard earth or rock reposing on clay or 
other bedding of similar properties, it converts the supporting 
layer into a semi-fluid mud, which opposes no obstacle to the 
sliding of the strata above. 
The upper part of the mountain which buried Goldau was 
composed of a hard but brittle conglomerate, called nagelfue, 
resting on an unctuous clay, and inclining rapidly towards the 
village. Much earth remained upon the rock, in irregular 
masses, but the woods had been felled, and the water had free 
access to the surface, and to the crevices which sun and frost 
had already produced in the rock, and, of course, to the slimy 
stratum beneath. The whole summer of 1806 had been very 
wet, and an almost incessant deluge of rain had fallen the day 
preceding the catastrophe, as well as on that of its occurrence. 
All conditions, then, were favorable to the sliding of the rock, 
and, in obedience to the laws of gravitation, it precipitated itself 
into the valley as soon as its adhesion to the earth beneath it 
was destroyed by the conversion of the latter into a viscous 
paste. The mass that fell measured between two and a half 
and three miles in length by one thousand feet in width, and 
X93) 
iiber die Untersuchung der Schweizerischen LHochgebirgswaldungen, 1862, p. 
61. 
Where more recent slides have been again clothed with woods, the trees, 
shrubs, and smaller plants which spontaneously grow upon them are usually of 
different species from those observed upon soil displaced at remote periods, 
This difference is so marked that the site of a slide can often be recognized at 
a great distance by the general color of the foliage of its vegetation. 
( 
19 
