THE LEYDEN ARGILLITE. 203 



cobalt, and no reaction foi* potash. The mass of kaoHn does not quite fill 

 the cavities. These occur north of the Devonian limestone in Bernardston. 



STRATIGRAPHY. 



The rock is crushed into sharp folds and finely coiTugated, and where 

 sandy layers are wanting- the i)riinary structure may be replaced wholly 

 by the cleavage; in other places it is brecciated and thrown into con- 

 fusion. Everywhere the strike and dip vary suddenly and within 

 wide limits. About N. 20° E. may be the average strike, and 60° E. the 

 average dip. 



A comparison of hand specimens, and especially of sections of the 

 three rocks, shows that the "argillite," while a distinct mica-schist, is far 

 less thoroughly metamorphosed than the schists in the Helderberg series, 

 and from this criterion alone one would consider it the newest rock in the 

 whole area. That it is newer than the mica-schists to the west and older 

 than the Helderbei'g series seems to me in the highest degree probable, and 

 also that the two older groups are Paleozoic; but I can find no very con- 

 vincing ground for their assignment to a definite horizon in the Paleozoic. 



BOUNDARY ON THE CONWAY SCHISTS. 



At Beaver Meadow, in the northeast comer of Leyden, one finds the 

 point of contact just at the foot of the mill dam. The black, barren argil- 

 lite has strike north to south, dip 70°-80° E., all the way up from Fall 

 River, a mile east, and often shows true cleavage. Here several thin, 

 rusty beds appear, and quite suddenly the rock becomes slightly coarser 

 and full of very small spangles and transverse crystals of biotite ; and three 

 thin beds of black limestone occur in quick succession. The boundary is 

 best drawn at the first bed of limestone, just at the dam, but for 300 feet 

 below the rock is black, fine-grained, finely double-corrugated, and differs 

 mainly in the minute mica spangling from the argillite lower down the 

 brook, and for a little way above this limestone much of the rock can 

 scarcely be distinguished from the argillite. It is, however, a little coarser, 

 rusty on cleavage faces, and spangled on transverse fractures. It is thus a 

 rather gradual transition, and President Hitchcock was often in doubt 

 about the existence of any boundary whatever. 



Exactly the same transition occurs between the two beds at all places 



