104 GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



prolonged far beyond the others. Highly magnified, they are reddish and 

 translucent. In other portions of the slide the characteristic enstatite- 

 serpentine occurs, with exactly the structure of fig. 3, PI. II. 



In the outer, bleached layer the lines of magnetite dust in the suture of 

 the chrysotile bands have disappeared and in the form of limonite have 

 soaked through the fibrous chrysotile. With further process of change the 

 latter loses its fibrous character, and every trace of the origin from olivine 

 disappears. At times also the olivine grains here retain their position in 

 the chrysotile network and their appearance, but have lost their bright 

 colors of polarization. The change of the olivine to tremolite seems to be 

 of earlier date, and has often advanced to the replacement of nearly all the 

 olivine. It appears to have occun-ed earlier than and independently of the 

 supei-ficial weathering, because the tremolite is found in equal abundance in 

 the fresh interior and in the weathered surface. 



Where the olivine is weathered, as in specimens taken from large 

 bowlders on the West Granville road, just where the glacial currents would 

 cany material from this bed, the specimens are not distinguishable from 

 coiTesponding specimens from the Pelham and Shutesbiuy beds (see p. 47), 

 and I had expected confidently to find the fibrous mineral to be antho- 

 phyllite, and indeed its powder gives fragments which foi' the most part 

 extinguish longitudinally, but at times other fragments extinguish with 

 an angle of 11° to 15° to the length, and show a single optical axis placed 

 laterally, exactly as in hornblende. 



A cleavage ^larallel to go P oo (100) seems to be unusuallv well devel- 

 oped, and most fragments rest upon it and so extinguish longitudinally, l)ut 

 show only one axis, as in hornblende. It is to be noted that this is from a 

 unique bed lying east of the main horublendic band. (See p. 89.) 



9. Pyroxene-serpentine. — Lowest bed, near Osborn's quarry. (Fig. 5, 

 p. 87.) The lowest bed at the margin of the north-south brook in the 

 woods below Osborn's quarry attracted my attention immediately as some- 

 thing quite unlike any other rock connected witli the serpentine series, to 

 which it manifestly belonged; indeed, unlike any rock with which I was 

 acquainted. 



The weathered surface is for the most part rough and warty, dirty 

 white, and covered with shining scales of talc. In some places this 

 layer, 5-10™" thick, is covered by a white, powdery layer of magnesite. 



