168 



MAEL BEDS. 



insoluble silicates in small quantities, as impurities. T. S. Hunt analyzed specimens from four different 

 marl beds in Vermont, with the following results : 



IViUiaiistoini. PcacJiam. Monltn. 



Carbonate of lime, . 



Carbonate of magnesia, 



Silica and traces of ox. iron and alumina, 



Water and vegetable matter, . 



82.6 

 2.5 

 2.6 



12.3 



89.0 

 4.2 

 1.0 

 5.5 



83.5 

 1.0 

 4.2 



10.5 



79.2 



2.4 



11.2 



13.6 



100.0 99.7 99.2 100.1 



All the marl beds in Vermont are known by the name of shell marl; because they have been chiefly 

 filled with, or formed of, the shells of molluscous animals. The beds occupy basin-shaped depressions, 

 which were once occupied by ponds, or they are found beneath existing ponds. Most of them are associated 

 with beds of peat and other forms of organic vegetable matter. Generally the vegetable matter overlies 

 the marl, because until a pond is nearly filled up, sphagnous mosses will not begin to grow upon its 

 surface. But when these do begin to vegetate, the water becomes so shallow, that very little lirne is 

 brought into it, and of course the peat takes the place of the marl. 



To illustrate the formation of beds of marl, we give a detailed description of one of the Vermont beds. 

 We quote the account from the. Second Report on the Geology of Vermont, by C. B. Adams, pages 148, 149. 

 It is but just to quote Prof. Adams, because he first described this bed for the benefit of the State. 

 He says : " The history of these beds of marl may be learned from the great deposit of Monkton Pond, 

 which is now in a stage of progress most favorable for illustrating their origin. This pond contains about 

 three hundred acres ; most of it, except the east portion which has a steeper shore, has the bottom deeply 

 covered with marl, which was probed to the depth of eight or ten feet, without finding the bottom. About 

 one third of the pond has less than five feet of water. The marl consists of shells more or less broken, and 

 slimy and reddish matter. The shells are of several species, which still exist in the pond. The accom- 

 panying section (Fig. 76) represents the several deposits, beginning at the north end of the pond with the 

 hill a, and extending about seventy rods into the pond ; c c, a muck bed ; n n, the pond ; e e, shell marl ; 

 o o, blue clay. On this section, one rod from the pond, the peat is two and a half feet thick ; beneath it we 

 find a few inches of marl ; and then blue clay, which was bored seven feet without finding the bottom. 

 Four rods from the pond the marl is but an inch or two in thickness. 



Fio.TG. 





" It is obvious that we have here a type of the usual process. First, the blue clay of the older pleistocene 

 was deposited over drift ; for although in this case we did not penetrate through it to the subjacent drift, 

 the known examples of its immediate superposition on the drift are so numerous, that there can be little 

 or no doubt ; then commenced the growth of the mollusca, which, although for the most part less than one 

 quarter of an inch in diameter, and occupying much less space after comminution, have accumulated to the 

 amount probably of 300,000 cords, or more than 6,000,000,000,000 of shells. 



" Meanwhile the vegetable deposit commenced not far from the margin of the pond, and is now advancing 

 into it over the marl, which however is still in progress thus showing us how, of two deposits superimposed 

 the one on the other, a part of the oldest portion of the upper one may be more ancient than the newest 

 part of the lower bed. 



" Since some of these beds are yet in progress, and others are entirely covered with several feet of peat, 

 and that with a heavy growth of timber, their ages must be various. That they are subsequent to the 

 blue clays of the older pleistocene appear from their very general superposition on these clays. In many 

 cases the process therefore must have commenced with the newer pleistocene period, or possibly somewhat 

 earlier, as it is not known they overlie the brown clay or sands. The process of filling up the ponds 

 in many cases also has been completed, and our views of the antiquity of this period must depend on 

 those which we adopt respecting the age of the overlying peat beds. 



