CHARLES A. DAVIS 



of all others and all precipitating calcium carbonate tubules. 

 The tubules are strong enough to serve as points of attachment 

 for other plants, and these add themselves to the little spheroid, 

 and entangle particles of solid matter, which in turn are held by- 

 new growths of the lime-precipitating Zonotrichia, and thus a 

 pebble of greater or less size is formed which to the casual 

 observer is in no wise different from an ordinary water- rounded 

 pebble. These algal calcareous pebbles show both radial and 

 concentric structure and might well be taken for concretions 

 formed by rolling some sticky substance over and over in the 

 wet marl on which they occur but for the fact that a consider- 

 able number of them show eccentric radial arrangement, and 

 that the shells of accretion are likewise much thicker on one side 

 than on the other, and finally, because the side which rests on 

 the bottom is usually imperfect and much less compact than the 

 others. The pebbles are characteristically ellipsoidal in shape. 

 The radial lines, noticeable in cross sections of the pebbles, are 

 considered by the writer to be formed by the growth of the fila- 

 ments, while the concentric lines probably represent periods of 

 growth of the plants, either seasonal or annual. Included within 

 the structure are great numbers of plants, besides the calcare- 

 ous Zonotrichia, among them considerable numbers of diatoms, 

 and it is probable that a large part of the algal flora of a given 

 lake would be represented by individuals found in one of these 

 pebbles. It is probable that to a certain extent they disintegrate 

 after the plants cease to grow, for they are never very hard when 

 wet. It is possible to recognize them, as lumps of coarser mat- 

 ter, even in very old marl, and the writer has identified them in 

 marl from Cedar Lake, Montcalm county, Mich., which was 

 taken from a bed a foot or more above, and several rods away 

 from, the lake at its present level. From the fact that these 

 pebbles have been found in four typical marl lakes in different 

 parts of Michigan (in Zukey Lake, by Dr. A. C. Lane, who was 

 struck with their peculiar character) and have been reported 

 from a number of others by marl hunters, it is probable that 

 they have a wide distribution in the state and are constant if not 



