OTTAWA VALLEY CONCRETIONS 625 



cult to escape this conclusion. It is hardly probable, however, that the 

 contrast between the chemical composition of the river and creek waters 

 is "the active factor in causing the scarcity in one area and abundance in 

 the other. A more potent cause is believed to be the difference in tem- 

 perature between creek and river waters. The two kinds of water con- 

 trast strongly in temperature except during the winter, when both 

 streams are sealed by ice. The lakes through which the Ottawa flows, 

 as well as its northern sources and greater depth, make its waters much 

 colder throughout the summer than those of Greens Creek. Thermom- 

 eter readings were made to test the amount of these differences on 

 August 15, with the results shown below: 



Surface temperature in Ottawa River near Greens Creek 72°-73° 



Surface temperature in mouth of Greens Creek 78°-80° 



The contrast in temperature between the water of the creek and river 

 in early spring is considerably greater than the summer contrast, as the 

 following figures, which were taken April 28, show : 



Temperature of water of Ottawa Ri\'er 34° 



Temperature of water, mouth of Greens Creek 54° 



Air temperature over river 46° 



Air temperature over creek noticeably higlier, but not measured 



The water of the Ottawa is derived largely from northern sources, and 

 for a considerable portion of the early spring its waters are notably colder 

 than those of small streams from the south like Greens Creek. The out- 

 flow of the waters of a relatively small stream of warm water into a river 

 with much colder water results in frequent fluctuations in the tempera- 

 ture of the water in contact with the bank below the creek mouth. 

 Whether the creek temperature or the river temperature will dominate 

 the narrow zone adjacent to the shore will depend on the relative stages 

 of water in the two streams as well as the strength and direction of the 

 wind at different times. There will be periods when the influence of the 

 creek will be blotted out by high water in the river, and vice versa. 

 These changes of temperature will be transmitted to the ground-waters 

 permeating the clays of the river bank and result in frequent and abrupt 

 changes in the lime-solvent powers of these waters by affecting their COg 

 content through increasing or lowering the temperature. Changes of 

 this character, it is believed, would stimulate the development of con- 

 cretions. 



Another factor in the control of conditions favorable to the formation 

 of these concretions is the diurnal change of temperature. Eiver and 

 creek bank sections partially or completely protected from the sun by 



XLI — Bull. Geol. Soc. Am., Vol. 34, 1922 



