142 FORMATIONS OF THE INTERIOR 
Bordering Lake Superior, red clays and marls, containing a large percentage of 
oxide of iron, underlie the Boulder Drift.* 
Many of the protrusive rocks composing the ranges of the interior of Wisconsin 
and Minnesota, are referable to some variety of granitic rock, each differing but 
little from the other in the ultimate elements of their composition ; varying, how- 
ever, considerably, in texture and grain of the component minerals. Some varieties 
are so fine, and contain so large a proportion of quartz, that they are hardly dis- 
tinguishable from a quartzite, especially in the more southern ranges. Others have 
a large proportion of felspar, of fine pink hues. 
At a few localities, hornblende enters largely into their composition, and the 
rocks pass into varieties of syenite or greenstone. This mineral enters also into 
the composition particularly of the narrow, wall-like dykes, which then assume, 
for the most part, the close texture and dark aspect of those varieties known under 
the general terms of Trap and Basalt. 
Crystalline schists and metamorphic slates flank the igneous ranges sometimes on 
both sides, sometimes on the north only. These often include or are in immediate 
connexion with augitic insinuations, which either appear in dykes, or as bedded 
trap, conformable to the stratification of the associate sedimentary strata. 
The trap of the Dalles of the St. Croix, however, exhibited in the annexed wood- 
cut, abuts immediately against the Lingula beds of the Lower Protozoic Sandstone ; 
TRAP, DALLES. OF THE SY. ..C ROT XxX. 
and broken fragments of these fossiliferous layers are even entangled in crevices and 
joints of the upheaved mass. The outburst appears to have passed through open 
fissures, breaking the continuity of strata, without tilting them into inclined planes. 
* The reader is referred, for the particulars on these heads, to the Reports of Dr. Norwood and Colonel 
Whittleséy. 
