i8o EDSON S. BASTIN 



sose the only potash minerals in the mode are biotite, microcline, and 

 orthoclase, while in the norm only orthoclase is present. 



II, ALBITE-PYROXENE-SYENITE 



These rocks form part of a series of greenstones which have 

 a more or less scattered distribution over an area of about 250 

 square miles among the islands of Penobscot Bay. The largest con- 

 tinuous area is on North Haven, where they occupy all but the 

 extreme southern parts of the island. From North Haven they 

 extend northwestward to Islesboro and the islands south of it; east- 

 ward they extend to the western part of Deer Isle; to the north 

 they reach the mainland in the vicinity of South Brooksville. 



The greenstone series consists of diabases, basic trachytes, and 

 albite-pyroxene-syenites, which in very many places show the char- 

 acters of surface volcanics in the occurrence of tuffs, breccias, and 

 amygdaloidal phases, and in the development of columnar parting 

 and "bolster" or "pillow" structure. Most of these rocks show 

 considerable alteration, but enough moderately fresh specimens 

 occur to place their original character beyond doubt. 



It is difficult to estimate the exact proportion which the albite- 

 pyroxene-syenite bears to the trachytes and diabases, but it prob- 

 ably constitutes nearly, if not quite, one-half of the greenstone 

 series. Its normal phase is massive, and it is usually associated 

 with the other greenstones and with the neighboring acid volcanics 

 in the form of dikes and sills. In a few cases, as will be described 

 later, it shows the characteristics of a surface volcanic. With most 

 of the other rocks of this region, it has been affected by regional 

 metamorphism, and in numerous localities a slight schistosity has 

 been developed. 



Intrusive phases. — The commonest type of albite-pyroxene 

 syenite is a massive to feebly schistose rock, usually dark green 

 in color, or showing a mottling of light and dark green, the light- 

 green portions representing somewhat altered feldspar areas, and 

 the dark-green the ferromagnesian constituents; in some of the 

 more feldspathic phases the feldspars are almost pure white. 



Upon microscopic examination, the texture is usually found 

 to be poikilitic. In some specimens this texture is rendered irreg- 



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