THE SILURIAN PERIOD. 391 



The Helderberg formation. — It has been customary in the past 

 to regard the Helderberg formation as a part of the Silurian 

 system, though its reference to the Devonian has often been sug- 

 gested. The fossils of the Helderberg formation are commonly 

 regarded as pointing to alliance with the Devonian faunas, rather 

 than with the Silurian. 1 By this standard, therefore, the reference 

 of the Helderberg formation to the Devonian system seems the 

 more appropriate. 



SUMMARY 



As in the case of all preceding systems of the Paleozoic, the greater 

 thicknesses of Silurian strata occur in the Appalachian mountain system. 

 Here the Medina and Oneida have a thickness of 1500 feet, the Clinton 

 of 2000 feet, and the Niagara and Salina of 1500 feet (central New York), 

 giving the Silurian system a thickness of about a mile, though this ia 

 far more than its average. In Maryland its thickness is between 2000 

 and 2500 feet. It is chiefly clastic bel^w, with limestone above. 2 Over 

 the interior, the system is relatively tMn, being measured by hundreds 

 of feet rather than by thousands. In keeping with these variations in 

 thickness, the Silurian along the Appalachian belt is largely of clastic 

 sediments of shallow-water origin, while that of the interior originated 

 in clear, though not necessarily deep water. During most, if not all, the 

 period, the Appalachian trough was shut off from free communication 

 with the interior sea, but connected with the Atlantic, perhaps by way 

 of the present Chesapeake region. The great thicknesses of strata in the 

 eastern mountain region were deposited in this Appalachian trough. 

 Since all or most of them were deposited in shallow water, they are 

 usually thought to point to the conclusion that the trough was sink- 

 ing, and that at a rate comparable to that at which the sediments 

 accumulated. This view may, however, need to be qualified by con- 

 siderations such as those suggested on page 257. So far as local sinking 

 took place, the accumulation itself, as already suggested, may have 



1 Clarke. Review of the Hercynian fauna, 42d Rept. N. Y. State Mus., p. 408. See 

 also views of Tschernychew, Am. Geol., Vol. XIV, 1894, p. 119; Schuchert, Bull. 

 Geol. Soc. Am., Vol. II, pp. 241-332; Williams (who advocates the opposite view), 

 idem, pp. 338-346; and Kayser, Geologische Formationskunde, zweite Auflage. 



2 Prosser, Jour, of Geol., Vol. IX, pp. 410-15. 



