PHYSICAL ORIGIN OF CERTAIN CONCRETIONS 453 



the Chemung concretions in southern New York. He supposes that 

 the fossils once occupied a definite horizon and have been pushed 

 upward and around the superjacent material which forms the body 

 of the concretion. Kessler and Hamilton/ after giving an analysis 

 of a certain gabbro and contained concretions, conclude with the 

 remark: "This similarity in chemical composition seems to denote 

 that the cause which set about the formation of the spheroids was 

 not a chemical phenomenon." The writers refer to the explanation 

 of Vogelsang^ in the type locality, Corsica; the latter suggested that 

 this concentric arrangement may be due to irregular areas of cooling 

 and contraction, Blake^ in referring to the concretionary structure 

 in white volcanic lava of Tucson, Arizona, takes the view that con- 

 centric structure, in that case, has been formed by deposition around 

 inclusions through action of permeating ground water. 



It is apart from the discussion here, however, to deal with the 

 concretionary and spheroidal structure of many eruptive rocks. The 

 present article is intended to consider the origin of certain types of 

 concretions common to sedimentary deposits only, and especially 

 those concentric nodules of argillaceous composition containing, in 

 many instances, noticeable percentages of calcareous and ferruginous 

 constituents, and to show that possibly physical forces have played 

 no little part in the forming of many of the common spherical, ellipti- 

 cal, discoidal, or irregular concretions in shales or clays. In some 

 cases concretions are known to have originated entirely from chemical 

 solution; an instance of this class is found in the well-known "loess 

 kindchen" of the loess deposits. These calcareous nodules are formed 

 often as incrustated deposits around roots and small plant stems. 

 Other occurrences present equal evidences of purely chemical origin. 

 But many of the more or less rounded nodules of concentric structure 

 and smooth surfaces have merely been aUuded to as products of an 

 affinity for like to like with no definite explanation as to how or why 

 they were so formed. The theory of attraction or affinity of like to 



^ H. H. Kessler and W. R. Hamilton, "Orbicular Gabbro of Dehesa Co., Cali- 

 fornia," American Geology, Vol. XXXIV, pp. 133-40. 



2 Sitzungsherichte der niederrheinischen Geschichte, Vol. XIX, p. 185, 1862, 



3 "Origin of Orbicular and Concentric Structure," Transactions of the American 

 Institute 0} Mining Engineers, Vol. XXXVII, p. 39. 



