I04 STRATIGRAPIIICAL GEOLOGY. 



mountain. The formation is five miles wide as it enters the White 

 Mountain district from the south, or along the route of Section VII. 

 Israel and Sandwich mountains in Sandwich are composed of this rock. 

 Guinea hill appears to contain a different rock, jutting into the eastern 

 edge of the porphyritic gneiss, so that the course of the range is irregular. 



Two sets of exposures farther east indicate the extension of a spur 

 easterly from Waterville along the base of the mountain range of Black, 

 Whiteface, and Chocorua. A little west of the usual path up Mt. White- 

 face this rock crops out, dipping 25° N. Perhaps the lower eighteen 

 hundred feet of the mountain, in ascending from McCrillis's house, should 

 be regarded as composed of this formation. The top rock is of sienitic 

 character. Little is known of the mountains between Whiteface and 

 Chocorua, so that the extent of the porphyritic rock easterly has not 

 been observed, but, as it constitutes the basis of everything in this part 

 of the country, it may turn up almost anywhere. Near the south-east 

 corner of Albany I found enormous boulders of this rock, enough to 

 make it clear that the ledge must be close by. A very small area is 

 therefore thus represented upon the map. 



Forty specimens illustrate these various areas in the museum, as indi- 

 cated by the catalogue. All rocks found within the porphyritic areas are 

 grouped together in the enumeration, although the igneous dykes are 

 evidently of later origin. It is probable that other bands of stratified 

 rock should be included, as will be developed by further study. The 

 typical field for the study of the porphyritic gneiss is the Merrimack 

 district. 



Bethlehem Gneiss. 



In 1 87 1 it became evident that between Carroll and Littleton there 

 existed an area of gneiss distinguished from all others by its peculiar 

 stratigraphical position and lithological character. The strike seemed to 

 run nearly east and west ; the strata stood almost upon their edges ; the 

 schistose character had nearly disappeared, so that we had previously 

 known the rock as granite ; and, lastly, a soft, greenish mineral, resem- 

 bling one of the hydro-micas, was disseminated through it. Hence I 

 ventured to separate this area from all the others then known, and gave 

 it the name of Bethlehem gneiss, from the town where it displayed itself 



