112 STRATIGRAPIIICAL GEOLOGY. 



granite, above the Lakes of the Clouds, upon the west side of Lafayette. 

 The connection between this and the larger area of Lake gneiss in 

 Landaff has not been made out, on account of the enormous coating 

 of drift concealing the strata west of the porphyritic gneiss. In the 

 museum, Nos. 315-320 illustrate these rocks. I should add that there 

 is a ledge of this gneiss between Bethlehem hollow and the Wing Road 

 junction, with strata dipping 80° N. 42° W. It belongs properly to 

 another field of description, the Ammonoosuc district. 



4. The Moxtalban Group. 



The limits of this formation have been set forth with sufficient exact- 

 ness in Vol. I, pp. 522-526, so as to render a repetition of the general 

 statements there made unnecessary in this place. The characteristic 

 rock of this formation is a feldspathic mica schist, carrying crystals of 

 andalusite or some closely related silicate. The variations are from mica 

 schist and quartzite on the one extreme, to well characterized gneiss on 

 the other. These schists are commonly quite ferruginous. The mica 

 occurs in small blotches, shining from light reflected at various angles. 

 The ledges of this variety commonly decompose readily. 



A not less important variety is a granitic gneiss, with very different 

 degrees of crystalline coarseness. The finer-grained rock often displays 

 no visible marks of stratification, though there is no reason to doubt its 

 sedimentary origin, and, for convenience, this is designated the Concord 

 granite. This rock is usually incoherent, tender, and quite friable after 

 decomposition has commenced. It is distinguished from the Lake gran- 

 itic gneiss by its fineness of texture. 



Along Mt. Washington river the gneiss contains many small nodules 

 of feldspar, so as to resemble the porphyritic variety of the Bethlehem 

 group. In Jackson there are a bed of siliceous limestone and layers of 

 compact feldspar. The eruptive members are quite important, consisting 

 of trap dykes, considerable masses of a porphyritic granite, light-colored 

 diorite, sienite, and large veins of quartz. The different ledges of this 

 rock among the White Mountains are well represented in the museum. 



Geographically, this formation may be divided into the main range of 

 the White Mountains east of the Saco river; the country to the east of 



