16 Gregory — Volcanic Rocks from Temiscouata Lake. 



From Point aux Trembles to the foot of the lake — a dis- 

 tance of about 10 miles — the rocks which outcrop are slates 

 and impure sandstones with rare tufaceous material. In 

 places the beds are much folded and traversed by faults of 

 small displacement. 



Petrography. 



The ordinary stratified rocks of the region present no pecu- 

 liarities which require detailed description. The volcanic 

 series consists of fine tuff and coarse amygdaloidal conglomer- 

 ate or breccia. 



Fine Tuff. — The fine tuff appears in the hand specimen as 

 a dense bluish-grey sandstone in beds a few feet in thickness. 

 Under the microscope its true character is revealed, and it is 

 found to consist of tiny fragments of andesite, broken crystals 

 of plagioclase and olivine, and areas of devitrified glass with rare 

 quartz grains and biotite shreds. The andesite has a hyalopi- 

 litic groundmass, which varies in amount of glass, so that some 

 fragments appear to be practically all glass with only a few 

 scattered laths of plagioclase. The larger crystals of andesine 

 have albite twinning and are largely kaolinized. The oliv- 

 ines are represented in the slide by areas of serpentine with 

 characteristic outlines and cleavages shown by dotted ore grains. 

 Iron ore is sprinkled quite generally through the section. 



Volcanic Conglomerate. — The volcanic conglomerate con- 

 sists of subangular pebbles of andesite and amygdaloidal 

 andesitic bombs, embedded in a finer matrix of red and green 

 grains of the same material. The pebbles range in size from 

 ■§• inch to 6 inches in diameter, and show in the hand specimen 

 phenocrysts of plagioclase, and occasionally epidote and augite. 

 The embedded bombs form a conspicuous feature of the rock. 

 They are quite spherical and are distinctly different in appear- 

 ance from the matrix. Amygdules, about the size of bird 

 shot, make up fully one-half of the bomb and are filled with 

 green balls of chlorite — rarely with calcite. Some of the larger 

 amygdules are seen to be occupied by chalcedony inside the 

 chlorite lining. 



Microscopic Examination. — The microscope shows the gen- 

 eral matrix of this coarse type to consist of crystals of feld- 

 spar, pyroxene and olivine, and fragments of andesite, devitri- 

 fied glass, and jasper. Iron dust, a small amount of calcite, 

 and a few shreds of biotite, are also present. The feldspar 

 crystals consist of a few orthoclases, and many rather small 

 andesines, with albite and carlsbad twins. The pyroxene crys- 

 tals are represented only by their outline, in a base peppered 

 with iron dust. The olivines are fragmentary crystals, and 

 largely replaced by serpentine and iron. The bulk of the sec- 



