ARGlLLITE AREAS 475 



North Lisbon. Multitudes of small garnets accompany the staurolites, 

 and in several ledges the garnets exceed 1 inch in diameter, accompanied 

 with large patches of masonite or chloritoid. There are very few out- 

 crops of the slate north of the line of the south branch, so that the de- 

 tails of this section, as shown in plate 42, figure 7, are important, illus- 

 trating the presence of erupted dikes and their effect upon the sediments. 



Under the bridge at North Lisbon lies the conglomerate already de- 

 scribed. For 600 or 800 feet to the east along the river no ledges are 

 visible, but the thick, white, fossiliferous limestone a few rods to the 

 south must represent the rock in place. On the north side there is an 

 outcrop of argillite supposed to lie east of the limestone along the line 

 of strike, and it is the first rock seen on the river's bank, 1,100 feet from 

 the bridge, with a dip of 45 degrees north, 32 degrees west. I think it 

 partly calcareous. Next is the calico conglomerate, 80 feet thick, having 

 a dip like that of the slate. To the east the slates appear again, with 

 higher dips, and disturbances produced by dikes of diorite (number 2 

 of Phalen), which has silicified the sediments by contact, changing it 

 into a quartzite. These altered strata are about 55 feet wide, including 

 the diorite. Following these are in order 50 feet of slate not much al- 

 tered ; a diorite dike 30 feet wide (number 4 of Phalen) filled with nod- 

 ules very similar to the elliptic bunches described by J. Morgan Clem- 

 ents in the Vermilion iron-bearing district of Minnesota* and b} 7 him 

 correlated with the pseudo-bombs of the Aa lava of Hawaii. 



The thickness of the slates seen along the bank to this point amount 

 to 146 feet. Next succeeds another diorite 304 feet wide. After that no 

 rock is visible for 50 feet, and then there are 165 feet of silicified slates, 

 followed by another Aa diorite 20 feet wide. Then follow in order 30 

 feet of slate, 85 feet of diorite, 20 feet of indurated conglomerate, accom- 

 panied by hornfels with altered slates for 320 feet. Beyond this point 

 the slates are more nearly normal up to 2,600 feet from the bridge. The 

 ledges become steep cliffs for an eighth of a mile farther, and there is an 

 elbow in the course of the stream, where is the locality of the large gar- 

 nets and chloritoids. 



These details were obtained in 1903. The locality was visited earlier 

 by Doctor Hawes, who was especially interested in the diorite or am- 

 phibolite, number 215 of the New Hampshire special collection. Other 

 specimens obtained from this section are numbers 152, 231, 232, 234, 

 235, and 245. That the diorite is not an altered sediment is proved by 

 its actual presence in dikes, its A a structure, and by the production of 

 contact phenomena. Why Doctor Hawes and myself should have both 

 failed to recognize the igneous nature of the diorites is singular, but our 



♦Monograph XL of the U. S. Geol. Survey, 1903. 



