38 NEW YORK STATE MUSEUM 



From an average of several determinations, the phosphorus may 

 be placed at about .5 per cent. 



The ore occurrence furnishes an interesting example of the re- 

 placement type of deposits. The associated rocks are crystalline 

 limestone and schists of Precambric (Grenville) age and Potsdam 

 sandstone. The Precambrics have a northeasterly strike and a 

 variable dip generally to the southeast. They are overlain by the 

 sandstone with a marked erosional unconformity at the base. The 

 dip and strike of the sandstone are subject to rapid changes from 

 place to place, though it usually lies more nearly horizontal than 

 the other formations. 



The limestone and schist form an interbedded series that is seem- 

 ingly conformable. Alternating belts of the two rocks occur across 

 the strike, the repetition being due probably to successive folds with 

 northeast-southwest axes. The schist is a quartzose graphitic 

 variety, finely laminated as a rule. It doubtless represents an 

 original silicious shale that contained considerable carbonaceous 

 matter. Secondary alteration, apparently connected with the process 

 of ore formation, has resulted in the development of chlorite from 

 the silicates, forming a chlorite schist. This is locally called ser- 

 pentine, but there is very little true serpentine present, so far as 

 observed. The chlorite schist is always closely associated with the 

 ore. 



In many places the sandstone capping is notably altered at the 

 base. It is decomposed to a soft iron-stained material, at times 

 carrying a fairly large percentage of hematite and grading into the 

 ore bodies below. 



At the Caledonia mine the deposits follow the contact between a 

 belt of limestone which constitutes the foot wall and an overlying 

 belt of schist. They have a pronounced irregular often sinuous 

 form, with bulges and pinches along the strike and dip. They fre- 

 quently branch and run off for some distance into the hanging and 

 foot walls. The Umestone along the contact has-been dissolved out 

 to form deep cavities and these may be filled with ore. Horses of 

 both limestone and schist are not infrequently encountered" in the 

 midst of the deposits. The workings do not appear to have pene- 

 trated anywhere the Potsdam, though it occurs on the hanging side 

 within probably 100 feet of the edge of the limestone. Where seen 

 on the surface above the workings it has a high dip to the east. 

 The tilting has been due probably to settling of the surface owing 

 to removal of the limestone by solution. 



