ADIRONDACK MAGNETIC IRON ORES 113 



definite angles. The large dike near Slope 15 is a mica diabase 

 containing abundant biotite in the place of augite which is the 

 normal ferromagnesian mineral. 



Geology of the ore bodies 



The ore bodies as previously stated are closely associated with 

 the augite gneiss, which is strongly developed throughout the dis- 

 trict and belongs to the Saranac formation. So far as the rela- 

 tions can be observed in mine workings and outcrops, they 

 appear to lie in immediate contact with the gneiss throughout most 

 of their extent, the only exceptions being at the Williams and 

 Dickson pits (and possibly the Parkhurst mine) where they are 

 bordered for some distance by schist. The latter rock is limited 

 to small bands included in the augite gneiss. 



The bodies consist of parallel zones of the gneiss, in which mag- 

 netite forms a relatively large proportion of the mass. The zones 

 possess a marked persistency along the strike and on the dip, 

 which with their small lateral dimensions gives them a prevailing 

 tabular shape. In structural arrangement they conform closely 

 to the foliation of the gneiss. Their geologic horizon appears to 

 be approximately a constant one, as they are alined in a general 

 northeast-southwest direction, parallel to the main strike of the 

 region. 



The borders of the deposits are not sharply defined. Stringers 

 and disseminations of magnetite extend into the gneiss for some 

 distance, forming zones of lean ore on either side of the main bodies. 

 This gradation is, however, a variable feature more evident in some 

 places than others. The gneiss itself shows no noteworthy change 

 as the ore bodies are approached. 



In character the deposits possess much uniformity throughout 

 their extent. The present main workings at Lyon Mountain are 

 practically continuous along the strike for a distance of 4000 feet, 

 with few variations noticeable in the occurrence or distribution of 

 the ore. In this respect they are in contrast with most magnetite 

 bodies which have been found to show frequent irregularities, 

 specially in form, from place to place. 



The ore is a mixture of magnetite and gangue minerals, the latter 

 mostly feldspar (orthoclase, microperthite, microcline and oligo- 

 clase), augite, hornblende and quartz. The different constituents 

 may be intermixed so as to show an even distribution, but more 

 frequently perhaps they have a rudely parallel arrangement, that 

 simulates the gneissoid structure of the wall rock. This is- par- 



