96 HISTORY OP THE EARTH.- 
52. Prop. VI. *ftfter the strata constituting the present 
crust of the globe had been deposited, and before the consolida- 
tion of the most recent secondary rocks, vast currents swept 
over its surface, and in some instances scooped out deep val- 
lies, and in others transported rocks to a distance from their 
original beds. 
Of the propelling power which put these currents in motion 
nothing is known with certainty ; whether for example they 
were rivers, conveying the waters drained from a continent to 
the ocean, or like the Gulf Stream, established in the ocean itself. 
The evidences of their existence are still extant in the marks of 
their ravages that remain. Nor is it meant to be asserted that 
all vallies have been formed by the action of currents. 
The attention of geologists was first called to the circumstances 
under which boulder stones are found dispersed over the north- 
ern parts of Switzerland, by Saussure. The Alps range along the 
southeastern border of that country. Mont Blanc, the highest 
peak, attaining an elevation of more than fifteen thousand feet, 
is a block of granite, in which talc and chlorite are substituted 
for mica. On the northwest, Switzerland is partly bounded, and 
partly traversed, by Mount Jura, presenting strata of secondary 
limestone. The two ranges are separated by deep vallies, in 
which flow, the Rhone, exnanding in one part of its course so as 
to form the lake of Geneva, and the Aar. On the top and along the 
sides of Jura are found huge blocks, which have apparently been 
torn from the opposite ridges of the Alps, and in some way or 
other transported across the valley. They have no connex- 
ion with, or resemblance to, the strata on which they lie, 
and are identical in composition, and structure, with the 
rocky masses that abound in the Alps. Respecting the man- 
ner in which their removal has been effected different opinions 
have been entertained : — that they had become enveloped in a 
body of ice, when the compound mass being lighter than an 
equal bulk of water, floated awa3 r , and eventually subsided into 
the situations in which the granitic blocks are now lying ; that 
they were blown into ihe air by the force which elevated the 
Alps and descended upon Jura ; that they were torn from the 
Alps, and carried down the sides of those mountains, with a ve- 
locity that caused them to roll up the side of Jura ; that there 
was once a continuous inclined plane reaching from the upper re- 
gions of the Alps, to the summit of Jura, along which they were 
rolled into their present beds, and that the intervening vallies 
were scooped out afterward by a current ; that the elevation of 
Jura was subsequent to that of the Alps, and that whilst it was 
yet on a level with the base of those mountains, the boulders were 
rolled down upon it, and afterwards elevated along with it. The 
alluvial shores of the Baltic present examples of erratic blocks, 
brought apparently from the mountains of Sweden and Norway. 
They occur also in the valley of the Ohio. 
