250 RODDY^CONCRETIONS IN STREAMS. [May 7. 



thickness. The laminse are not equally compact throughout their 

 thickness, but are open and porous within and quite solid without. 

 A polished section of any concretion exhibits many concentric 

 ellipsoidal layers with the nucleus nearly always eccentric and the 

 successive layers with a greater thickness on the one side and two 

 ends than on the other side. The thickness of the successive 

 laminse in any one direction out from the nucleus is nearly uni- 

 form. In other words, along any radius the inner layers are just 

 as thick as the outer ones. When found in place in the stream 

 where the concretions have not been disturbed for a long time, the 

 down side laminse are invariably a little thicker than those on the 

 upper side. This indicates that the greater growth is downward. 



In appearance and structure, the concretions of the Little Con- 

 estoga are very similar to the " Lake Balls " from Lake Canan- 

 daigua, New York, so vividly described by Dr. Clarke, under the 

 name of " Water Biscuits." They are also somewhat similar 

 though much larger in size to the oolitic sands found forming in 

 great numbers in the waters of Great Salt Lake by A. Rothpletz 

 and traced by him to the agency of blue green algse. 



Where Found. — Upon recognizing the importance of a thorough 

 study of the Algoid concretions, I began a systematic search in all 

 parts of the Little Conestoga as well as in other streams of both 

 Lancaster and York Counties, Pennsylvania. My search showed 

 that these objects abound in all parts of the Little Conestoga nearly 

 from source to mouth. But no other streams in this part of the 

 state have so far yielded any specimens. Those found in the sand 

 bar in Lake Canandaigua near the mouth of Sucker Brook are 

 probably also of stream origin, and I feel confident that a careful 

 search in the brook would reveal at least some, if not many, of the 

 concretions. Substances somewhat similar in composition occur in 

 other lakes than Canandaigua though they do not have the con- 

 cretionary form. Thus laminated reef-like accumulations of Algoid 

 origin occur in Round Lake, New York, while marly or tufa- 

 ceous deposits have accumulated for ages and are still forming in 

 many lakes in Michigan, Wisconsin and Indiana. The tufa and 

 thinolite described by Russell as forming in Pyramid Lake, Nevada, 



