ADIRONDACK MAGNETIC IRON ORES . 1 5 



being interbedded with limestone and hornblende, mica and 

 pyroxene schists. The St Lawrence county quart zites contain 

 feldspar and mica, but are not so graphitic as those of Essex 

 county, where they have been exploited. 



The quartzites are no doubt ancient sandstones that have been 

 hardened by recrystallization of the quartz particles; they may be 

 considered, therefore, to represent the extreme silicious phase of 

 Precambric sedimentation. 



Gneisses of undetermined relationship. The recent work in the 

 Adirondack region has disclosed the existence of certain gneisses 

 of obscure character. While more detailed investigation may 

 resolve them into elements which can be classed with the igneous 

 or sedimentary series, they have been found so far to have no well 

 defined connection with either. 



Saranac formation. The principal area of these gneisses seems 

 to be on the northern borders in Clinton and Franklin counties. 

 Professor Cushing has described a belt that extends along the 

 Paleozoic contact for a distance of 70 miles. The rocks are mainly 

 red acid gneisses, composed of alkali feldspar, which is usually 

 microperthite, and quartz, with small amounts of hornblende and 

 biotite. They are thus mineralogically related to the granites, but 

 differ from the latter in their textures which are often finely gran- 

 ular or without the definite arrangement that characterizes igneous 

 rocks in their original state of consolidation from a molten condi- 

 tion. Besides the acid gneisses a gray variety consisting of pyrox- 

 ene and feldspar (orthoclase and plagioclase) and dark hornblende 

 gneisses or amphibolites occur as bands or larger masses. Of the 

 Grenville rocks there are very few exposures throughout the entire 

 belt. The gneisses as a whole correspond in composition quite 

 closely to a series of igneous rocks grading from granites through 

 syenites and diorites to gabbros, though the comparison has not 

 been substantiated fully by chemical analyses. 



Professor Cushing is inclined to regard them as an older series 

 than the recognized intrusives and has proposed to group them 

 collectively as the Saranac formation, a name suggested by their 

 occurrence along Saranac river. Concerning their possible posi- 

 tion among the Precambric rocks of the Adirondacks, Professor 

 Cushing points to the similarity which they show to the basal 

 gneisses in other regions and more specially the so called Ottawa 

 gneiss of Canada; while he seems to favor the view thai they 

 represent the original floor on which the Grenville rocks have been 

 deposited, he does not regard the proofs for this explanation to 

 be fully established. 



