378 The American Geologist. Doccmber, i89T 
out exposure to that streani. Notable exceptions to such dis- 
tribution of the exposures are the prominent ridges, one to 
three miles distant from the river on either side, rising con- 
siderably above the surrounding drift. On the accompanying 
luaps the location and surface extent of these rocks are given. 
The total area of their outcrops is about two square miles. In 
general, the occurrence and characters of the igneous rocks of 
this region have already been described by geologists.* 
Many of these early descriptions are accurate and followed 
by logical conclusions. Present opinions (jf geologists are 
summarized in the following statements: 
The igneous rocks of the St. Ci-oix vallev represent the southwestern 
extension of the volcanic flows and sedimentary strata known in the 
lake Superior region as the Keweenawan or Copper Bearing series. Irv- 
ing insists upon the identity in essential characters of the igneous rocks 
of Keweenaw point and those of the St. Croix valley, t and this is con- 
sidered by geologists the true correlation. 
Keweenawan sedimentaries have not yet been observed in these 
southwesterly areas. The rock has been called most frequently a mel- 
aphyr. A serrate outline^ of some of the outcrops and a step-like ap- 
jjearance of others has been referred to as indicative of the dip of these 
beds. A system of jointing planes is regarded as evidence of separate 
and perhaps numerous flows of molten matter. The vertical planes of 
this system represent an essentially basaltic fracture approximately 
perpendicular to the cooling surfaces: while the more persistent though 
somewhat irregular planes, more nearly horizontal, suggest lines of sep- 
aration between successive flows. The dip of these planes, which is 
fairly uniform throughout the area, is, in the vicinity of St. Croix Falls, 
an average of 15 degrees W. by S.§ 
Differences in physical and mineralogical character corresponding 
roughly with the above mentioned separation planes, have been noted 
by the earlier geologists but not fully described. Conglomerates in 
contact with the volcanic rocks at numerous places are regarded as be- 
longing to the next geologic age — the Cambrian. || 
*Owen: Geol. Survey of Wisconsin, Iowa and Minnesota. 1852. 
Chamberlin (Strong): Geology of Wisconsin, vol, iii, 1880. 
Upham: Geol. and Nat. Hist. Survey of Minn., Final Report, vol. ii, 
1888 
Winchell: 10th An. Rep. Geol. and Nat. Hist. Survey of Minn., p. 120. 
Kloos and Streng: Neues Jahrbuch filr Min.. Geol. und Paleont., 
1877. (Translation in the 11th Report of the Minnesota Survey.) 
Kloos: Zeitschrift d. Deutsch. Geol. Gesells, 1871. (Trans, in 10th 
Minn. Rep.) 
Irving: U. S. Geol. Survey, Monograph v, 188.3. 
tMonograph v, U. S. G. S., 1883, pp. 239, 240. 
JGeology of Wisconsin, vol. in, 1880, p. .369. 
^Compare Geology of Wisconsin, vol. iii, 1880, p. 120. 
[[Geology of Wisconsin, vol. in, 1880, p. 117. 
