IQIS-] 



RODDY— CONCRETIONS IN STREAMS. 



247 



Though these structures, as I shall show later on, are without 

 doubt due to Algoid agency in the stream waters, it may be well 

 to premise the full discussion of their origin by somewhat com- 

 plete descriptions of their characteristics as to form, size, struc- 

 ture, etc. In this way the attention of botanists and geologists 

 will be directed to their study and distribution, so that their signifi- 

 cance as agents of rock formation and the flora, responsible for 

 their growth, may be fully worked out. 



Size and Shape. — The concretions both in the stream and in 

 the deposits vary in size from peas to masses nearly a foot in di- 

 ameter (see Fig. i). The latter size is not very common in the 



Fig. I. A group of the concretions showing their size, shape, surface 

 appearance and color. No. i is 7>4xio inches; No. 2 is about 5 inches in face 

 diameter and 3 inches thick; No. 3 is 8x7x5 inches. The two smaller con- 

 cretions above are t3'pical, both in color and surface appearance, of growing 

 specimens. 



stream but many large concretions occur in the deposits probably 

 because the smaller ones after deposition in land forms have been 

 carried away in solution by percolating waters leaving only the 

 larger forms. In the flood deposits in Kendig's Woods thousands 

 of the concretions when I found the deposit last summer measured 

 nearly a foot in length and six inches or more in transverse 

 diameter. 



