RETORT OF THE DIRECTOR AND STATE GEOLOGIST 



rS7 



rock. In consequence, foliation is no criterion for the correla- 

 tion of different areas of gneisses. 



As a matter of fact, there is abundant evidence that the gneiss 

 formation is not a unit, but rather a complex, so far as age is 

 concerned; while the differenl parts have so much in common as 

 regards origin and history that their separation is practically 

 impossible. Tims the gneiss is treated as a unit in mapping, 

 though iis complex character is fully recognized. However, such 

 treatment seems justifiable on theoretic as well as practical 

 ground, since it is probable, first, that the differences of ages 

 among the gneisses are relatively slight, and second, that the 

 rocks had a common source. These points have been treated in 

 previous reports at such length that farther reference to them 

 may be omitted for the present. 



As elsewhere in this part of the state, the gneisses are the most 

 widespread of the pre-Cambrian rocks, occurring in large irregu 

 lar areas around the belts of schists and limestones, and also in 

 minor amount within the latter. It is often difficult to draw a 

 definite line of demarcation between the two formations, since, 

 in some cases, the schists approach very closely to the gneisses 

 in composition and texture. 



As these two formations, together with the granite, were the 

 chief object of field study, their distribution and relations are 

 bes1 considered before passing on to the Potsdam. 



Beginning at the northwestern limit of the region studied, 

 which corresponds with the international boundary line, Wells 1 

 and Grindstone islands afford an epitome of the geology, as all 

 the formations encountered are shown on them. 



The northeast half of Wells island consists chiefly of the schist 

 formation, of which two members appear. On the east is a \ « t\\ 

 gneissoid member, so difficult to distinguish from the true granite 

 gneiss that their distribution as shown on the map must be re- 

 garded as quite conjectural. • 



On the w est side of the island, beginning at Westminster Park, 

 is a high ridge of white vitreous quartzite with just enough feld- 



'Ov Wellesley. 



