36 XEW YORK STATE MUSEUM 



Of these the maximum figures are the more probable. These 

 strata thus constitute a formation of considerable thickness, with a 

 fauna essentially the same throughout, and distinct both litholog- 

 ically and faunally from the formations above and below, from each 

 .of which it appears to be set off by an erosional break (discon- 

 formity). They appertain to the Theresa or "passage" beds as 

 originally defined by Professor Cushing, and are the equivalents 

 roughly of the upper portion which on the Theresa and adjacent 

 quadrangles he and Doctor Ulrich have more recently referred to 

 the Tribes Hill.^ Considering the absence here of the rich Tribes 

 Hill fauna, the isolation of these northern New York strata from 

 the typical beds of that horizon in the Mohawk valley, and their 

 very different lithologic constitution and local subdivisions, a dis- 

 tinctive local name is needed for them. No satisfactory type locality 

 having as yet offered, the name here used owes its selection chiefly 

 to the fact that it was not likely to be preoccupied, and it should 

 be permitted to lapse as soon as a permanent one can replace it. 



Beekmantown (Ogdensburg) Dolomite 



The rocks occupying the area that was assigned to the " Calci- 

 ferous " (now called Beekmantown) on the state geologic map 

 are, in our region, fine-grained, firm, fairly uniform, gray to drab 

 dolomites and dolomarenites usually with a rather dull, velvety 

 surface when fresh and often a coarsely ribboned or thin-lamellated 

 appearance when weathered. Sand grains are absent except in 

 restricted layers, and while the rock is sometimes slightly argillace- 

 ous it is seldom silicious. The magnesian content varies ; most lay- 

 ers effervesce poorly, but some strongly and some not at all in dilute 

 acid.^ The sand-crystal cleavages so frequent in beds below 

 (Theresa and Bucks Bridge) are here wanting or at least very rare. 

 Like the Llockport dolomite of western New York, this rock usually 

 consists of tiny sandlike crystal grains of dolomite in a calcite or 

 dolomite cement, and like that rock it furnishes an excellent build- 

 ing stone. 



Masses and nodules of cleavable crystalline calcite occur more 

 or less commonly, as also exactly similar ones of dolomite, dis- 

 tinguishable only by failure to foam with dilute acid. These masses 



' N. Y. State Mus. Bui. 145, p. 64-66. 



^The larger the percentage of magnesia (dolomite instead of calcite) 

 in a limestone, the less readily it will effervesce (foam) with weak acid. 



