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POPULAR SCIENCE REVIEW. 
tributary of the Isere and ultimately of tbe Rhone. Crossing 
the Rhone at Culoz, the railway is carried along the expanded 
valley forming the lake of Bourget to Chambery, and thence, 
in the same direction, without any works of magnitude, but 
through scenery of very great beauty, till it enters the main 
valley of the Isere. Proceeding up that valley for a short dis- 
tance and crossing the river, it turns aside to enter the valley 
of the Arc, passing Aiguebelle and traversing a narrow strip of 
granite rock till it reaches St. Jean de Maurienne, a small 
town in Savoy, celebrated for its numerous and frequent earth- 
quakes, and situated at the extremity of a projection of this 
granite rock to the east, penetrating the altered jurassic rocks 
which flank the granite both on the east and west. The rocks 
here consist of a large series of gypsums and clay, including 
deposits of iron ore and some veins of galena. A little beyond 
St. Jean a calcareous rock is crossed, believed to be the repre- 
sentative of the nummulitic series largely developed in southern 
Europe, and of an age recognised as lowest tertiary. After 
crossing a long strip of this limestone the jurassic rocks re- 
appear, and beyond St. Michel they are found to be altered 
and greatly metamorphosed, putting on an aspect so generally 
characteristic of old rocks, that the geologist not well accus- 
tomed to the Alps and to Alpine geology would be easily 
deceived by them. A very extensive series of these metamor- 
phosed oolites forms almost the whole mass of the two western 
divisions of the Alpine system, called the Cottian and the 
Graian Alps, separated by the valley of the Dora Riparia, and 
except where interfered with by some patches of serpentine 
and euphotide, they are remarkably connected and uniform 
throughout. The tract of country separating France from 
Italy in this part of Europe affords one of the lowest and most 
accessible of the numerous passes across the crest or culminat- 
ing axis of the Alps. This occurs near the Mont Cenis, and 
over the col or pass known by that name is constructed one of 
the many magnificent roads for which Europe is indebted to 
the first Napoleon, who, imitating Hannibal in ancient times, 
had previously brought his army from France to Italy. 
The Mont Cenis pass, though long a difficult and troublesome 
mountain path, seems to have been known and used from time 
immemorial. It has only within the present century become a 
great highway connecting north-western Europe with Italy, and 
is on the whole well adapted for that purpose, its highest point 
being only 6,890 feet above the sea. It may be crossed, with 
comparatively few exceptions, during the whole winter, and it 
is not so subject to destructive avalanches as many of the passes. 
The general width between the two principal valleys of the 
Arc and the Dora is here only twelve English miles, but the 
