Notes on Monhegan Island. — Lord. 337 
developed. This latter mineral (of both generations) is poikil- 
itically intergrown with pyroxene, magnetite and feldspar 
crystals, which in thin sections gives it a very characteristic 
mottled appearance. The hornblende is of dark brown color 
and strongly pleochroitic: parallel band C=chestnut brown, 
parallel a=yellow. Absorption isc > b> a 
The poorly developed crystals vary in size from i mm-2 cm 
and have well defined prismatic cleavage, against which a max- 
imum extinction (c:c) of 14° was determined. The above phy- 
sical properties correspond almost exactly with those of a 
brown hornblende from the hornblende-gabbro near Pavone, 
Upper Italy, described by van Horn* and regarded by him to 
be syntagmatitet plus a normal-orthosilicate molecule 
(R-Si04) with the constitutional formula: (H, K, Na )- 
(Mg. Fe, Ca). (Al, Fe> Si. do. 
It will be remembered that in discussing the mineral com- 
ponents of the olivine-noryte, attention was called to the fact 
that in many cases olivine and feldspar were more or less com- 
pletely replaced by secondary amphibole (tremolite and actino- 
lite) accompanied by chlorite and sometimes biotite. This high- 
ly metamorphosed rock may be termed, altered, or amphibol- 
ized noryte in contrast to the amphibole bearing gabbro-dio- 
ryte, which is genetically more directly connected with the 
gabbroitic phase of the noryte proper, and can only be dis- 
tinguished macroscopically from this rock by its dark 
green or grayish green color and variable coarseness of 
grain (see p. 332). 
With the, aid of the microscope the gabbro -dioryte is seen to 
be composed of basic feldspar, green hornblende, magnetite, 
biotite, apatite and frequently diallage more or less uralitized. 
The alteration of diallage to green hornblende (uralite) by 
paramorphism has, thanks to Williams, Chester ;]; and others, 
been so well described that it needs no special mention here. 
It may, however, be of interest to note that many of the sec- 
•tions contained, besides this fibrous amphibole, another of 
massive structure and idiomorphic form. These crystals are 
*Pet. Untersuchungcn ueber die nortische Gestiene der Umge- 
bung von Ivrea in Oberitalien. Min. u. pet. Mitth., XVII B., 5 Heft, 
1897, pp. 19-23. 
tSee R. Scharizer, neues Jahrb., 1884, II, p. 142. 
JBull. 59, U. S. G. S., pp. 25-27. 
