Gregory — Volcanic JiocJcs from Temiscouata Lake. 17 



tions examined is formed of fragments of pyroxene-andesite 

 of a type sparingly porphyritic with small stout pbenocrysts 

 set in a groundmass, having in some places a glassy base, in 

 others being made up almost entirely of ragged feldspar micro- 

 lites, arranged with flow structure. The fragments of denitri- 

 fied glass present show occasional cusps, which represent 

 expanded steam cavities. No close line, however, can be drawn 

 between the fragments of glass and the fragments of andesite 

 containing a few feldspar microlites in a glassy base. The 

 dark-red iron dust is not generally distributed through the 

 rock, but is present along the cracks of the olivine crystals, 

 and it also gives color to the glassy fragments. 



The Amygdaloidal Bombs. — The section cut from one of 

 the amygdaloidal bombs reveals the composition and structure 

 of an extrusive basic andesite. The minerals present as pbe- 

 nocrysts are plagioclase, pyroxene, olivine and iron ore, with 

 infiltrated quartz and calcite in the amygdules. There is also 

 a considerable development of minerals of the chlorite group, 

 perhaps largely delessite. The plagioclases are short, rather 

 square crystals with ragged ends and strongly-marked albite 

 and pericline twinning. Many of them are bent and curved and 

 irregularly broken. In composition they are between andesine 

 and labradorite. Augite occurs as stout crystals always 

 partly decayed and replaced by chlorite or epidote, or rarely 

 by calcite. Olivine crystals are about as abundant as augite 

 and show their characteristic parting along cracks now filled 

 with iron. Serpentine has generally replaced the olivine. 

 Iron ore occurs in a few ragged grains. The groundmass is of 

 feldspar microlites usually untwinned and with little evidence 

 of flowage. The structure was hyalopilitic, but the glass fill- 

 ing the interstices is devitrified and stained brown with iron. 

 The steam cavities now filled and converted into amygdules, 

 though very abundant, have exerted little influence on the 

 arrangement of the feldspars in the groundmass — a fact which 

 suggests that these cavities formed in part before any crystal- 

 lization of feldspars had occurred. 



All the amygdaloidal cavities are filled with secondary pro- 

 ducts which have a uniform arrangement. They are lined 

 with a coating of a malachite-green mineral of the chlorite 

 group, arranged in radial forms. Most of the smaller cavities 

 are completely filled with this substance and appear on the 

 rock surface as embedded green pellets. The larger cavities 

 have the chlorite lining fractured, and fragments of it have 

 floated in toward the center, leaving means of access for later 

 infiltrations. Similar phenomena have been observed in certain 



A.M. Jour. Sci. — Fourth Series, Vol. X, No. 55. — July, 1900. 

 2 



