Elements of geology, or, The Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments elementsofgeolog00lyel Year: 1868 654 LAVAS OF MADEIEA. [Ch. XXIX. it may quite as often arise from the circumstance that such accumu- lations of loose ejected materials have been exposed from the first to greater waste in regions where the snows melt suddenly, and where the winds are most violent. A dense covering of turf and shrubs, the most effective of all preservatives against pluvial degradation, cannot readily be formed in such mountainous an
Elements of geology, or, The Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments elementsofgeolog00lyel Year: 1868 654 LAVAS OF MADEIEA. [Ch. XXIX. it may quite as often arise from the circumstance that such accumu- lations of loose ejected materials have been exposed from the first to greater waste in regions where the snows melt suddenly, and where the winds are most violent. A dense covering of turf and shrubs, the most effective of all preservatives against pluvial degradation, cannot readily be formed in such mountainous and stormy regions. Some few lavas in Madeira have a singularly recent aspect as com- pared to others which are covered with a considerable depth of vege- table soil. I allude particularly to the lava currents near Port Mouiz, one of which is as rough and bristling as are some streams before alluded to in Palma (p. 641) of historical date. I am indebted to M, Hartung for the annexed drawing of lava at Port Mouiz, which I did Fi<r. 709. Surface of lava near Port Moniz, $T. ~W. point of Madeira; frc a. â Channel traversing the lava. [rawing by M. Hartung. not visit myself. It is traversed by a channel, a, like one of those already described (p. 637). For how long a period such characters may be retained is uncertain, so much does this depend on the min- eral composition of the rock. Some of the lavas of Auvergne, of prehistorical date and certainly of high antiquity, are almost as rug- ged ; so that this freshness of aspect is only a probable indication of a relatively modern origin.
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