GEOLOGY AND SALMON.

Lyttelton Times, Volume XXXI, Issue 2602, 7 May 1869, Page 3

GEOLOGY AND SALMON.

(From Land and Water.)

The dip, lie, and condition of the rocks seem greatly to affect the nature and character of rivers. If strata are horizontal, or nearly so, water flowing over them cuts a channel consisting of a series of waterfalls impassable to salmon. For if the rocks, as is usually the puse, consist of alternations of hard and soft beds, the soft beds are worn away, forming not only a waterfall, but an under-cut full, as the splash of the descending water and the back wash denude the soft bed under it. These are nearly certain to be impassable, more especially ns from the lie of the beds it is rarely possible that a pod] can be cut at their base. If the rocks are all about tbe same hardness a rapid without pools is formed, up which no fish can go. If rocks dip at high angles the river usually is more favourable for fish to pass, for although there way be waterfalls, yet in general they are so modified that fish cau pass, except when the rocks dip up stream, which forms ns bad a fall as any in horizontal strata. If the strata strike across the stream, there is often a perpendicular Tall, caused by a bard bed adjoining a soft one; but this may counteract itself, as below the fall in the soft bed such a deep pool may be

cufc out by the trituration of the blocks and fragments of rock carried down by the water, that the fish can jump over. If the beds lie obliquely to tho course of the stream there is rarely an impassable fall, but more generally a rapid, consisting of a series of pools and slides, up which fish can shoot. Moreover, in rocks that have a high inclination there usually are more or less rolls, curves, and contortions, and these, combined with the high dip, are generally most favourable, as usually | they are certain to form rapids, consist- j ing of pools alternating with either | slides or low leaps, or, perhaps both j combined. It may here be observed j that a fall over which salmon can jump j is generally know as “ leam ” ( Anglic& j leap), seemingly a corruption of “leam na bradaun ” (Aw/lice, the leap of a salmon, or a salmon-leap.) It has been noted of England, Ireland, and Scotland, that good salmon rivers are rare if they occur among rocks of the newer formations. This general remark seems to be accounted for by the previously-stated facts ; because in the newer formations the beds are usually horizontal, or nearly so, while in the rocks older than the carboniferous limestone, the strata not only generally dip at a high angle, but also they are almost sure to be affected with curves, flexures, and contortions. In other respects the newer rocks ought apparently to be as favourable to the salmon as the older; but if the rivers in a tract of country are full of impassable falls they cannot be good salmon streams, as the first place that the salmon cannot pass causes all the water above it to be unproductive. This is the case in many of the rivers in a country where the rocks are horizontal, while many rivers that flow through high and contorted strata are productive for miles, the fish often being able to ascend to the foot of the mountain torrents. In some places they even can go up too high, for in a torrent, where piles of debris are carried down each 'flood, to which fish can get, much spawn is destroyed, being covered by the sand and gravel that are swept down. Salmon apparently cannot ascend or descend through subterranean passages, although trout seem capable, as the latter fish will be found in subterranean pools and other places to which they could not get except they bad gone through an underground passage; whether the salmon cannot live in such places, or that they cannot get through the intricate passages has not been proved; but it is a patent fact that much valuable ground in Ireland is lost on account of it lying above a subterranean river. A desideratum for a good salmon river seems to be that it should have lakes connected with it, and the nearer one of these is to its mouth so much, apparently, the better. Many small rivers, on this account, are much better than larger ones, the latter being without lakes. However, there are exceptions to this general rule.