336 CEMENTS, LIMES, AND PLASTERS. 



diately a series of natural forces were set in operation which tended 

 to remove this lake. Of the two common methods of lake disappearance, 

 one has but little to do with our present subject, but the other is closely 

 connected with the history of marl deposition. The first method is 

 the gradual deepening of the outlet by the action of its own current, 

 resulting in the draining of the lake. The second is the filling up of 

 the lake basin by deposits of sand, clay, marl, muck, and peat. 



Origin of marl deposits. The exact cause of the formation of marl 

 deposits has been the subject of much investigation and discussion, 

 particularly in the past few years, since these deposits have become 

 of so great economic importance. For details concerning this dis- 

 cussion, reference should be made to the papers listed on pages 346-347, 

 especially to those by Davis and Blatchley. In the present place only 

 a summary of the main facts and theories in regard to the forma- 

 tion of marl deposits will be given. 



Marl deposits in their present form and position are due directly 

 or indirectly to glacial action, on which account these deposits occur 

 almost exclusively in that portion of the country which was covered 

 by ice during the Glacial epoch. The glaciers aided in the formation 

 of marl deposits in two ways: first, by furnishing a large supply of 

 finely ground calcareous material from which surface waters could 

 take up lime carbonate in solution, and, second, by forming the lake 

 basins in which these waters deposited their burden of lime carbonate 

 as marl. The processes followed in the formation of marl deposits 

 may be outlined with some confidence, though certain of the steps are 

 still subject to discussion. 



Rain-water, though theoretically pure before approaching the earth's 

 surface, takes up a considerable percentage of carbon dioxide in pass- 

 ing through the atmosphere. When it reaches the surface, therefore, 

 such rain-water is in reality a very dilute form of carbonic acid (H^COs), 

 and as such is capable of attacking limestone, taking the lime car- 

 bonate (CaCOs) into solution in the form of calcium bicarbonate 

 (CaH 2 (C03)2). Practically all natural water, including the percolat- 

 ing ground-water as well as spring and stream water, is therefore able 

 to change itself with lime carbonate if some source of that compound 

 presents itself. In the present marl districts such a source is not far 

 to seek, for from New York to Michigan limestones cover a large pro- 

 portion of the surface. The glaciers during their advance over this 

 area ground up vast quantities of the surface rocks, leaving the pul- 

 verized debris in the form of deposits of limey gravels and limey clays. 



Surface waters running through such areas, or underground waters 



