The lightning plays around the summits of the mountains in a constant succession of brilliant flashes, whilst the thunder is prolonged through the deep ravines and distant valleys, until the echo of the one peal and the crash of another blend together in one never-ending roll. Heavy falls of snow often accompany these storms, and the condition of travelers crossing the passes during one of them is most distressing. Unable to advance or to retreat, they halt and wait, in momentary fear of being hurled over the mountain sides. Blinded by snow, and by the vivid flashes, they dare not proceed; the ledges also are, perhaps, so narrow, that if they would they could not turn the mule round to retrace their steps. In such a position as this, men have been compelled to remain during many hours in places where the thermometer falls every night in the year below freezing point, and where the most intense darkness--whilst it fails to hide the real dangers, conjures up imaginary ones, which multiply all the horrors of the scene. There are some portions of Upper Peru which are yet comparatively unknown to Europeans. This is especially the case with that part of it which has declared itself an independent republic, under the name of Bolivia. Though possessing a coast town on the Pacific of considerable extent, with several good harbors, yet its singular formation precludes ranch intercourse with other countries. Between the Andes and the sea is a broad belt of barren desert; a sand plain in continual motion. This is traversed by a few small rivers; which, though very shallow and often dry during the summer months, render the strips of soil through which they pass extremely fruitful. Beyond this desert, the most inaccessible chain in the Andes rises and forbids approach to the fair country enclosed within. On the summit of this chain is the celebrated mountain Potosi, now nearly exhausted of its treasures: the town is situated in a district wholly destitute of vegetation. Passing from the Ceno de Pasco through the town of Larma, we enter the valley of Janja, and shortly find ourselves in a country presenting a strange contrast to the one we have just left. A succession of the most fertile valleys in the world. As the ascent of the mountain commences from the low country, the sandy desert disappears. A rich coat of lucerne spreads over the sheltered hollows. Vines and olives appear in the vales. The sugar-cane, the banana, the guava, and numberless tropical fruits flourish. At the height of eight, and sometimes ten thousand feet, Los Vales of Bolivia are covered with the most luxurious vegetation. Forest-trees of gigantic size are thickly spread over the mountains. The cereals, which live a sickly life down by the sea, appear in these lofty valleys in full vigor: including maize, quinna, rice, barley, with occasional patches of wheat, though of this last the chief supply is imported out of Chili.