160 W. A. Bell — Mississippian Formations of 



with a cone-in-cone structure. Certain beds in these zones 

 are abundantly rich in leperditioid and beyrichioid ostra- 

 cods, Spirorhis, and the scattered scales and dermal 

 bones of palseoniscid fishes. The ostracod beds are of a 

 more argillaceous character, and are more restricted in 

 their vertical distribution, being mainly confined to the 

 middle part of the Horton. 



The higher beds of the upper Horton comprise a thick 

 accumulation of finely siliceous shales associated with 

 thick interzones of non-laminated, or but poorly lami- 

 nated, argillo-arenaceous deposits that break with a char- 

 acteristic hackly fracture, and that weather frequently to 

 variegated light greenish and butf -yellow colors. An ex- 

 traordinary feature presented by these beds on their 

 exposed bedding surfaces is a polygonal system of crack- 

 ing that might readily be mistaken for true sun-cracking. 

 Careful inspection, however, reveals the presence of car- 

 bonaceous traces of a dichotomously branching system of 

 rootlets, and not uncommon association with casts of up- 

 right tree stems, denoting clearly that they are fossil 

 soils. It is these early soil zones that lend to the deposit 

 its greatest interest. Although they may be observed 

 in their best development in the upper Horton (they are 

 particularly well displayed along the Cambridge shore), 

 their occurrence is widespread both vertically and later- 

 ally, and they are eminently characteristic of the whole 

 deposit. In several hundred feet of strata in the upper 

 Horton, fifty-six well marked soil zones were noted, of 

 which seven had associated with them abundant upright 

 tree stems. Additional soils and upright stems were 

 present in the ostracod-bearing division below. The 

 abundance of these upright stems in the horizons in which 

 they occur is evident from the fact that ninety-six were 

 counted in a plot 150 X 15 feet. The stems are small, 

 rarely exceeding 11 inches in diameter, and usually not 

 more than a foot of the height can be traced. As regards 

 the rootlets in the soil zones, their casts and imprints are 

 indistinguishable from the rootlets of Stigmaria ficoides 

 of the Coal Measures. It is a striking fact, however, 

 that no creeping ^* rhizomes'' comparable to Stigmaria 

 were found anywhere in the Horton formation, while sev- 

 eral upright stems had directly attached to them rootlet- 

 like appendages that branched dichotomously. The ab- 

 sence in the Horton of true Sigillaria and the association 



