GENERAL AND IXTERIOB DISTRIBUTION Cvii 



As a further consequence of its geological antiquity, involv- 

 ing degenerative chemical changes and a long-continued leach- 

 ing, the soil of this lower giaciation has become an extremely 

 fine-grained, light-colored cla^^ which, when compact, sheds water 

 almost completel}^ but which washes into the streams as a fine 

 detritus that remains persistenth^ in suspension and renders the 

 waters very urbid for a long time after a rain. Standing pools, 

 indeed, never become even approximately clear. So persistent 

 is this turbidity, due to very finely divided matter in suspension, 

 that the chemists of the Water Survey find it almost impossible 

 to free the water wholl}^ from suspended solids even bj^ repeated 

 filtration. Furthermore, this soil has a definitel}^ acid reaction, 

 to which is due a notable physical difi"erence between the soils 

 of this area and those of the later glaciations west and north of it. 

 A surplus of lime in a soil coagulates or granulates it, causing its 

 ultimiate particles to cohere in larger granules, while in an acid 

 soil this effect is entirely wanting. This lack of granulation in a 

 very finel}" divided soil increases, of course, the permanent 

 muddiness of its waters as compared with those of the areas in 

 which lime in the soil renders it alkaline. 



The acidity of this southern soil seems not to be of a kind or 

 amount to afi"ect the surface waters sensibh^ and directl}^ since 

 the water samples from this region analyzed by the State Water 

 Surve}^ show a soft water, slightl}^ alkaline, and chemically un- 

 objectionable as a medium for fishes. 



CLASSIFICATION AND USE OF ECOLOGICAL DATA 



That these conditions are a part, at least, of the cause of the 

 phenomenal distribution of southern Illinois fishes maj^ be shown 

 by a comparison of our ecological data for the fishes of the two 

 lists — one composed of those adapted to the conditions of the 

 lower Illinoisan giaciation and the other of those avoiding them. 

 In the organization of the data of our collections of Illinois fishes, 

 those concerning the character of the water bodj^ in which collec- 

 tions were made were classified in a way to show the number of 

 collections of each species taken from each class of situation. 

 By reducing these numbers to ratios of frequency of occurrence, 

 we have a means of exhibiting the preference of species with 

 respect to the situations in which each occurs. Piiyiephales 

 notatus, for example, was found twenty times over a muddy 

 bottom to thirty-four over a bottom of mud and sand, and to 

 forty-six over a bottom of rock and sand. Aphredoderus 



