394 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



various natural watercourses. As all the other external conditions 

 (climate) of these brooks are the same, the reason for this phenomenon 

 can only be found in the difference of the geological strata from which 

 these brooks take their origin; it is a fact that a difference in these 

 geological strata goes.hand in hand with the appearance of these brooks 

 and with the varying occurrence of fish. The waters which come from 

 speckled sandstone (in the Spessart and Rhon Mountains) are clear and 

 transparent, and contain trout, without any artiiicial aid, as far as i^ar- 

 ticles of such sandstone reach, while brooks which spring from shell- 

 lime are invariably void of fish. This fact is all the more remarkable, 

 as those waters which spring from speckled sandstone contain fewer 

 dissolved particles than any other water iu Germany. Of dissolved 

 carbonic acid only a faint trace can be chemically discovered, and dis- 

 solved lime is found only in very small quantities. These i)articles, 

 which are found in considerable quantity in shell lime brooks, form the 

 most important substratum of all organic life. Aquatic plants live on 

 carbonic acid, which is fixed, as it were, by the lime contained iu the 

 water. The aquatic flora of the speckled sandstone brooks is also un- 

 usually i)oor in individuals and species compared with the rich flora 

 of the shell-lime brooks. Water-plants, however, are indirectly an essen- 

 tial condition of the well-being of trout, by serving as food for water- 

 snails, the larvse of various insects, and small crustaceans. This 

 investigation, when extended over a wide area, also showed that in 

 other parts of the country waters containing lime were very rich in 

 fish. Great credit is due to the late Professor Weith, of the Univer- 

 sity of Zurich, for having shown in the report of the Swiss Depart- 

 ment in the International Fishery Exposition of Berlin (1880), that 

 among the Swiss waters (both lakes and brooks) those were ahvays 

 richest in fish, especially salmonoids, which contained relatively the 

 largest quantity of dissolved carbonic acid and lime. For this reason 

 it could safely be predicted that the attempt to introduce young trout 

 into the shell-lime brooks of Franconia would prove successful. The 

 result has proved the correctness of this prediction. In several brooks, 

 which for a period of three or four yeai's were stocked with young trout, 

 the fish flourished to an extraordinary degree; there are more fish than 

 in equally strong watercourses of the sandstone formation, and owing 

 to the abundance of good natural food, the fish grow much more rapidly 

 than in other waters, so much so that, compared with them, trout from 

 the speckled sandstone waters must be termed decided failures. The 

 shell-lime brooks, therefore, fulfil the first condition of the well-being of 

 trout, viz., lo supply good and sufficient food for fish of every age. The 

 case is difl'erent, however, as regards the second condition, viz., the fa- 

 voring of propagation. In this respect the trout (and in fact all 

 salmonoids) require very peculiar conditions, because the eggs, after 

 having been laid and become impregnated, need a very long time (a 

 quarter of a year and more) till the little fish are hatched. During all 



