DESCRIPTION OK PORT HENRY TYPE LOCALITY. 249 



of aluminous and siliceous sediments. The derivation of the serpentine 

 of the ophicalcite from pyroxene illustrates this impure character, and 

 the copious development of the original white pyroxenic mineral is a 

 proof of tlic presence of siHca and magnesia. 



With this introduction we may pass to describe in greater detail the 

 two typical exposures in Port Henry and Keene, using them as illustra- 

 tions of the others. The former is on the edge of tlie mountains and the 

 latter, which is some 20 or 25 miles to the northwest of it, is well within 

 the great areas of anorthosites. 



THE PORT IIEyRY TYPE LOCALITY. 



General Description. — The most accessible exposure is near Port ITenr}^, 

 which is in the township of Moriah, Essex county. In size this town- 

 ship is al)out eight miles north-and-south by nine miles east-and-west. 

 It is chiefly an upland valley, cut off from lake Champlain by a high 

 ridge of gneiss and gabbro running north-and-south, but with a break at 

 Port Henry, tli rough which the valley slopes, down to the water. On 

 tlie south a high east-and-west ridge of gneiss incloses it, and to the west 

 the great ridges of the Adirondack? begin to appear. Though broken by 

 many minor elevations, the valley character is well preserved. North of 

 Port Henry the limestone series forms the country-rock along the lake 

 for about three miles, being beautifully exposed together with the in- 

 truded gabbros in the railway cuts. The series appears on the west 

 side of the eastern bounding ridge of mountains. It is also known in 

 the central portion, south of Mineville, and again in the southwestern 

 j)art tlie characteristic coarse white marbles and green ophicalcites are 

 well developed. 



The Moriah valley itself is to a very great extent buried under drift, 

 but it is ])rol)al)le that the limestone series underlies nearly all of it, and 

 that the hills o( gneiss project, chiefly from faulting and the erosion of 

 the former caps of limestone. The undoubted frequency of faults and 

 tlie great disturbances this region has undergone make the stratigraphic 

 relati(»ns very dillicult to determine, but it is evident that the limestone 

 series is limited to the valleys, and that as we reach the higher ridges its 

 cliaracteristic strata fail and are rei)laced by light colored, rather massive 

 micaceous gneiss, dark Ijasic hornl)lende-gneiss and intruded sheets of 

 gabbro. 



Cruii-i-scctions. — Figures 8, 4 and 5 represent cross-sections just north of 

 Port Henry. They are one mile each from west to east and have the 

 vertical and horizontal scales the same, and for their toi)ograi)hic out- 

 lines are enlarged from the Port Henry sheet of the United States CJeo- 



XXXV-Birr.i.. «Koi,. Hor, Am.. Vol. fi. 18»4. 



