FRAGMENTATION — BAKBELL 293 



RELATIONS OF ISOSTASY, BASIC INTRUSIONS, AND CRUSTAL FOUNDERING 



Let US turn now to the opposite side of the problem, that is, to 

 the possible relation of igneous intrusions of high density to regional 

 subsidence. 



Granites, the siliceous or acidic end of the igneous series, have, 

 on the average, 70 per cent silica and a specific gravity of 2.70. 

 Gabbros, the dark, coarse-grained, basic intrusives of the same chem- 

 ical nature as basalt, average 50 per cent of silica and a specific grav- 

 ity of 3. The extremely basic rocks, pyroxenites and peridotites, 

 have an average silica content of 45 per cent and a specific gravity 

 of about 3.30. The density of these rock types is therefore seen to 

 range through 20 per cent. 



At Sudbury, Ontario, a great intrusion of gabbro shows after 

 ages of erosion as a great lens 36 miles long and 17 miles broad. 

 The thickness ranges from half a mile to 2 miles, and in form it 

 is a saucer-shaped body concave upward.^^ It is the shape of the 

 mass that is so interesting here, since it appears that the floor sub- 

 sided under it during or after the intrusion. Such a saucer-formed 

 body differs from a laccolith, in which the intrusion domes up the 

 cover. This fundamental distinction in form appears to be related 

 to the higli elensity. Being denser than the surrounding rocks, the 

 igneous mass has settled down. 



The Duluth gabbro which underlies the southwest part of Lake 

 Superior is on a far larger scale than the Sudbury intrusion and 

 accounts for the synclinal structure of this part of the Lake Superior 

 Basin. In the Middle Keweenawan, basaltic lavas were poured out 

 on the surface of the earth over the western half of the basin to a 

 maximum of more than 20,000 feet in thickness, the bottom sub- 

 siding as the successive floods welled forth. Thus the original basin 

 structure of Lake Superior was established at this remote time, but 

 the present depression occupied by the lake waters seems to be of 

 recent geologic origin, as though, at a time of relatively recent 

 crustal disturbance, the great weight suspended here in the crust 

 had, after an age-long rest, dragged it down another stage. The 

 glacial ice occupied this ready-made trough and by its weight may 

 have deepened it a little further. Although the extrusives are so 

 thick, the intrusives as shown in the Duluth lopolith are known to 

 be much thicker. They presumably overlie still deeper and larger 

 reservoirs of igneous rock, offering a yielding base to the lavas 

 above, as otherwise depression of their floor would hardly have gone 

 forward so readily during the period of igneous activity. 



" = lopolith of Grout. Amer. Journ. Sci., 46, 516-522. 1918. 

 24034r-29 20 



