BY A TARN SIDE. 6 



fested, a bountiful supply of life-forms. The fine 

 whirling dust will have peopled the water with 

 infusoria or diatomacese, the green scum of algae 

 will mantle the surface, the larvae of many aquatic 

 insects will sport in the water. But give us a 

 genuine tarn for natural history purposes one 

 whose existence extends perhaps beyond the historic 

 period. There are many ways in which such tarns 

 may have been formed. In mountainous districts 

 they may, perhaps, be the result of glacial agencies 

 which scooped out the rock-basin of the tarn itself. 

 They may occupy the hollows of the land, as in some 

 parts of Cheshire, where the dissolving of the strata 

 of rock-salt beneath, and their removal as brine- 

 springs, has caused the overlying rocks slowly to 

 settle down. Or, still more common, and far more 

 widely distributed, they may have . been formed, as 

 we have frequently discovered they have been, in 

 chalk or limestone countries, where the superficial 

 di-ainage has dissolved away portions of the soluble 

 rock, and thus formed " sand-pipes." In the hollows 

 thus left, whenever water has been capable of being 

 held, you may depend on a genuine tarn. It may 

 be centuries old, surrounded by low bushes, covered 

 with aquatic plants a veritable microcosm, in which 

 the " battle of life " has long ago been settled, and 

 the animals and plants have adjusted themselves to 

 each other's needs. What myriads of agencies have 

 been at work to stock a little pool ! The reptiles 

 and fish may have been originally brought, as ova, 



