GEOLOGY OF OGDEXSBURG 29 



This statement is regarded as necessary here since, in following 

 these beds on to the Brier Hill and Ogdensburg sheets, and tying 

 them up with Professor Chadwick's work on the Canton sheet, it 

 is found that they have greatly thickened, have changed somewhat 

 in character, and that apparently another formation has wedged in 

 between the lower and upper parts of the formation, as shown in 

 the Thousand Islands region. Furthermore the upper part, the 

 Tribes Hill, is shown very sparingly and erratically in the district 

 here, but reappears on the Canton sheet in even greater force than 

 on Theresa. We seem to be dealing with the thin, near-shore 

 edges, of several formations, very similar lithologically, whose 

 thorough discrimination and description is going to be a very diffi- 

 cult matter, and can not yet be successfully attempted, chiefly 

 because of the lack of well-preserved fossils. 



Description. As here mapped, the Theresa formation contains 

 at least two different elements. The lower portion of the formation 

 consists chiefly of thin-bedded calcareous sandstones and sandy 

 limestones, blue-gray and very hard when fresh, but weathering 

 rapidly to yellow-brown, porous rocks, such as usually appear in 

 outcrop. Occasional thin beds of white sandstone, usually brown- 

 spotted after the fashion of the uppermost beds of the Potsdam, 

 occur with the others. This material has a general thickness of 

 from 25 to 30 feet, and seems to be the exact equivalent of the 

 typical Theresa of the Theresa quadrangle. It is on the same 

 horizon and looks the same. But we have found no fossils what- 

 ever in it in this district, though it should hold L i n g u 1 e 1 1 a 

 actiminata if our correlation is correct. The zone is, how- 

 ever, seldom well exposed here, and good opportunity for search 

 for fossils is therefore lacking. 



Above this thin-bedded zone there is a recurrence of thick-bedded 

 material, with much increase in the amount of sandstone. This is 

 quite foreign to our experience with this passage bed zone else- 

 where in New York. In general tliere is a steady diminution in 

 the amount of sandstone, going up in the section. But here the 

 increase is so marked that, in many exposures, it would be an ex- 

 ceedingly difficult matter to discriminate these beds from the Pots- 

 dam, were not the horizon definitely determined. Midway of the 

 zone is a solid mass of sandstone, 20 feet thick, which is so promi- 

 nent a feature in the geology of the district that we have given it a 

 separate mapping. Professor Chadwick had independently recog- 

 nized this sandstone on the Canton quadrangle and has suggested 



