198 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



in the Eastport area of Maine. The Silurian outpourings were especially 

 voluminous and, where identified, are chiefly andesites and basalts, al- 

 though acidic varieties have been noted. Volcanism was again widespread 

 and voluminous in the Devonian. Lower Devonian volcanics are known 

 in the Gaspe Peninsula, in northern New Brunswick, and in the Lake 

 Ainslie area of Nova Scotia; Upper Devonian lavas have been noted in 

 the St. Andrews region of New Brunswick near the Maine border. The 

 Devonian volcanics are mostly andesites. 



The Carboniferous was unremitting in volcanic activity, and consid- 

 erable amounts of lavas and tuffs were extruded. The Mississippian 

 rocks of the Magdalen Islands contain basaltic lavas and fragmentals, and 

 those of the Hampstead area of New Brunswick contain rhyolite. Penn- 

 sylvanian rocks in the St. John region of the Bay of Fundy contain ex- 

 trusive and intrusive rocks, and the Bonaventure formation along the 

 north shore of Chaleur Bay contains amygdaloidal basalt flows. 



Lavas, chiefly andesitic and basaltic, and graywackes and arkoses with 

 sandstones, shales, and limestones compose a stratified assemblage typical 

 of the eugeosyncline of Kay (1951). 



Intrusive Rocks 



Intrusive rocks are widespread in Nova Scotia and New Brunswick. 

 They are granites and associated differentiates, that accompanied the 

 Acadian orogeny at the close of Lower Devonian time. The granites are 

 exposed over much of the southern upland of Nova Scotia, and the central 

 highlands of New Brunswick. 



A belt of ultrabasic plutons, now largely serpentinized, extends through 

 the Quebec Appalachians from Vermont to Gaspe, and their intrusion is 

 thought to have accompanied the Taconic orogeny. See Fig. 8.29. 



Many dikes and sills are mentioned in the literature, and these prob- 

 ably relate to the volcanic series. 



A group of eight small intrusions in southern Quebec form the Mon- 

 terigian Hills. The most westerly is Mount Royal at Montreal. Except for 

 one, they lie along a curved line that extends easterly from Montreal. Five 

 of them rise well over 1000 feet above the surrounding plain; the others to 

 heights of 600 to 700 feet. The five westerly ones intrude the flat-lying 



beds in front of Logan's line, and the three easterly ones cut the folded 

 and faulted Paleozoics east of the line. According to Caley ( 1947 ) : 



Brome and Shefford Mountains are thought to be unroofed laccoliths, or per- 

 haps parts of a single laccolith still covered by sedimentary strata in the 2% mile 

 interval of lower land between the hills. The remaining hills appear to be vol- 

 canic necks with nearly vertical walls. 



The age of the intrusions is Devonian or younger. Evidence for this, in addi- 

 tion to that supplied by the St. Helen Island breccia, is afforded by Yamaska, 

 Shefford, and Brome Mountains, which lie within the folded Appalachian re- 

 gion. The intrusive masses show no effects of deformation, and hence must 

 have been intruded after the last folding that affected this region in Middle 

 Devonian time. It has also been noted that in the Monterigian intrusive rocks 

 pleochroic haloes surrounding crystals of zircon and titanite are invariably 

 poorly developed and immature. In this they resemble those in Tertiary intru- 

 sive rocks, whereas in certain Devonian granites haloes are numerous and prom- 

 inent. The suggestion has, therefore, been advanced that the igneous rocks of 

 the Monterigian Hills may be as young as Tertiary. 



STRUCTURES 



Unconformities 



The Paleozoic section is replete with unconformities and conglomerates 

 which indicate intermittent orogeny from place to place over a long 

 time. 



A coarse conglomerate of Lower Cambrian age containing large gra- 

 nitic boulders rests on rocks of the same material as the boulders near 

 St. John, New Brunswick. Lower Ordovician black slates with a basal 

 conglomerate and some interbedded impure quartzite or graywacke 

 overlie unconformably the Caldwell group of the Cambrian in the Thet- 

 ford area of southern Quebec. Limestone conglomerates of Lower 

 Ordovician age occur in places from Vermont through Quebec to New- 

 foundland and have been interpreted as the result of local slipping and 

 breaking up of limestones, just deposited, along the sea bottom by earth- 

 quakes in a zone of crustal deformation. 



In Nova Scotia, a coarse conglomerate and grit of Middle Ordovician 

 age overlies beds of Lower Ordovician age. On the north side of Chaleur 

 Bay, coarse conglomerates of Middle Ordovician age made up largely of 

 the Proterozoic (?) Macquereau rocks, rest on the Macquereau. In the 



