Pirsson and Washington — Geology of New Hampshire. 499 



needles 2-3 mm long (hornblende) ; tough., with a somewhat 

 hackly fracture ; on exposed surfaces altering to a leather 

 brown crust dotted with black hornblende needles. 



Microscopic. — Hornblende and plagioclase essential ; iron 

 ore and apatite accessory ; calcite and chlorite secondary. 

 The hornblende is present in two generations ; the larger 

 average about l'50 mm in length by 0'40 mm in breadth, the 

 smaller about 0'25 by 0*05. There are many gradations 

 between them. Except in size they are alike in other respects, 

 rather long columnar in development with 110 and 010 well 

 developed, good terminals lacking. It is strongly pleochroic ; 

 c and h rich leather-brow^ a pale ocher yellow ; absorption 

 marked c = h > a. Angle of extinction c on c = 18°. The min- 

 eral includes a little iron ore and is very fresh and unaltered 

 save in a few spots where it is changed to chlorite. The plagio- 

 clase, as the average of several determinations by Michel-Levy's 

 method shows, is a labradorite of about the composition Ab 7 An 8 . 

 Its form is that of slender laths whose dimensions are similar to 

 those of the small hornblendes mentioned above. Both Carls- 

 bad and albite twinning occur. A few sporadic larger crystals 

 of the same characters but in size like the larger hornblendes 

 were observed. The smaller feldspars are often coated with 

 films of an isotropic substance which also fills minute spaces. 

 Its nature could not be ascertained, but presumably it is anal- 

 cite and its association with calcite leads to the supposition 

 that it is secondary. 



The iron ore is in small grains *05 to *10 mm in diameter, 

 peppered everywhere through the rock, sometimes agglom- 

 erated into larger lumps and often beading the edges of the 

 hornblendes. Apatite occurs in minute needles of the charac- 

 ter usual in such rocks. 



Calcite is found liberally sprinkled through the mass in 

 very minute pieces occupying little angular interspaces between 

 the feldspars and other minerals ; in these cases it does not 

 appear as an ordinary alteration product since the adjoining 

 minerals are fresh, but rather as an infiltrated material, if it 

 is not indeed an original component. In a few places, how- 

 ever, it is agglomerated into masses which from their outlines 

 and general appearance are evidently pseudomorphs of a former 

 mineral, apparently augite, a few crystals of which about the 

 size of the larger hornblendes were originally present. This 

 seems to indicate that probably the calcite is to be regarded 

 as secondary. These sporadic augites are the only minerals in 

 the rock which have suffered any considerable alteration — the 

 others being in good condition. 



Mode. — The quantitative mineral composition as determined 

 by the Kosiwal method is as follows : 



