SOME UNUSUAL ROCKS FROM MAINE i8i 



ular by the great variations in the size of the feldspar individuals, 

 while in others it has been obscured or destroyed in the develop- 

 ment of a schistose structure or in the weathering. The feldspars 

 are striated and usually form rather short prisms, most of which 

 range from ^ to i|™™ in length; in many of the shdes they 

 are remarkably fresh. They are shown to be albite by their low 

 extinction angles, and by the index of refraction, which is uniformly 

 equal to, or less than, Canada balsam. The pyroxene, which nor- 

 mally forms a poikihtic matrix for the feldspar laths, is pale pinkish- 

 yellow in color, with a hardly noticeable pleochroism, and shows 

 extinction angles ranging up to 45, but usually under 40°. These 

 angles are found in both augite and diopside, but the fact that 

 the angles never exceeded 45° in the half-dozen or so slides studied, 

 as well as certain considerations as to chemical composition to be 

 brought out later, points to the non-aluminous diopside rather than 

 to an aluminous pyroxene. Magnetite is the only other original 

 constituent which plays an important role; it occurs in irregular 

 grains, which sometimes assume skeleton forms, and may then 

 partially inclose feldspar crystals. 



Near the head of Southern Harbor, on the island of North Haven, 

 the syenite is pecuhar in the possession of scattered feldspar pheno- 

 crysts which are occasionally | inch in length. Relatively fresh 

 representatives of the common massive type are found on Hard- 

 head Island and at the southern end of Spruce Head Island. 



Exrustive phases. — Amygdaloidal phases of the syenite were 

 found on the southern end of Bare Island. A part of Mark Island 

 and the greater portion of Beach Island are occupied by a phase 

 of the greenstones characterized by the development of very ragged 

 surfaces on weathering, and by the possession often of a purplish 

 tint which is not characteristic of the greenstones elsewhere. In 

 certain places the weathering reveals a distinctly tuffaceous charac- 

 ter. In the field these were looked upon as flows and tuffs, and 

 this is borne out by the microscopic examination, which shows them 

 to be a very fine-grained phase of the albite-pyroxene-syenite series. 

 The rock shows a diabasic texture with laths of albite, varying con- 

 siderably in size, but mainly minute; between them is chlorite, epi- 

 dote in small grains, and somewhat altered magnetite. 



