Powell: OBSERVATIONS ON SOME CALCAREOUS PEBBLES. 77 



of Mr. Davis given above is the most probable one for their 

 formation. 



A study was made of the pebbles which had been preserved 

 in 5 per cent, formalin. Sections cut with a sharp scalpel were 

 placed in a weak solution of hydrochloric acid until decalcified, 

 after which they were immediately mounted in glycerine jelly. 

 The pebbles were found to be composed of a densely interwoven 

 mass of filaments of which the most common type was those of 

 Schizothrix fasciculata Gomont. The trichomes were 1.4 to 3 

 mic. broad. The pseudocysts were equal in length to diameter 

 of trichome or twice as long, 1.2 to 3.5 mic. long. A species 

 of Calothrix, two species of Cosmariimi, a JVostoc and numer- 

 ous diatoms were also present in the pebbles. 



In June, 1901, Miss Daisy Hone collected some small peb- 

 bles at Point Douglas, Minn., on a steep bluff side overlooking 

 the Mississippi river. Being high above the water they were 

 not supplied with moisture and seemed perfectly dry at the time 

 of collection. These pebbles had an outside layer of calcareous 

 material, which, when decalcified, showed the presence of an 

 alga, a species of Scytonema. 



Bibliography. 



1. Murray, G. Phyc. Mem. 74-77. 1895. 



2. Thiselton-Dyer, W. T. Ann. Bot. 5: 225. 1890-91. 



3. MacMillan, C. Minn. Plant Life, 41. 1S99. 



4. Davis, C. A. Journ. Geol. 8: 502. 1900. 



5. Davis, C. A. Journ. Geol. 8: 495. 1900. 



Explanation of Plates. 

 Plate XVI. (in Part). 

 Figure 8. A section of a pebble showing stratified layers, nat. size. 

 9. A filament of Schizothrix, x 600. 



10. Calothrix sp. Detail of an older filament, X 335- 



11. Same. Portion of thallus showing method of branching, x 64. 



12. Same. Young free filaments, X 200. 



Plate XVII. 

 Figures 1-5. Calcareous pebbles, one half natural size. 



1. Pebble showing smooth surface. 



2. One showing a rougher surface. 



3. One showing a still rougher and more porous surface, probably 

 wave-worn. 



4 and 5. Hollow interiors of two pebbles are shown. 



