486 CHARLES A. DAVIS 



slightly cream colored, coarsely granular to finely powdery, very 

 loosely coherent and effervescing freely in acids. On dissolving 

 it particles of vegetable and other organic and insoluble matter are 

 found scattered through the solution. 



The ultimate source of this material, except the vegetable 

 matter, is, undoubtedly, the clays of glacial deposits and like 

 disintegrated rock-masses. These clays are rich in finely 

 divided limestone and in the softer rock-forming minerals, some 

 of which contain calcium compounds. Percolating water, con- 

 taining dissolved carbon dioxide, the so-called carbonic acid gas, 

 readily dissolves the calcium and other metallic salts up to a 

 certain limit. The water with the dissolved matter in it runs 

 along underground until an outlet is reached and issues in the 

 form of a spring. This, in turn, uniting with other springs 

 forms a stream which runs into a lake, carrying along with it 

 the greater part of its mineral load. If the amount of carbon 

 dioxide contained in the water is considerable, some of it will 

 escape on reaching the surface, because of decrease of pressure, 

 and with its escape, if the saturation point for the dissolved 

 mineral matter has been reached, a part of this matter must be 

 dropped in the form of a fine powder, as the water runs along 

 over the surface. Theoretically, then, some, if not a great part 

 of the dissolved matter, should be thrown down along the 

 courses of the streams which connect the original outlets of the 

 water from calcareous clays and lakes where marl occurs, and 

 we should find the marl occurring in small deposits along these 

 streams wherever there is slack water. Moreover, we should 

 expect the waters of these springs and streams to show more or 

 less milkiness on standing exposed to the normal pressure of the 

 atmosphere at usual temperatures. Actually, however, none of 

 these phenomena have been noted, and we infer that there is not 

 a large amount of calcium dioxide, and not an approach to the 

 saturation point for calcium bicarbonate, in the springs and 

 streams feeding marly lakes. 



We are then left, among others, the following alternatives, 

 explanatory of marl formation: (i) The marl is not being 



