84 The Auiericdii (ieoUxjist. August, 1895. 
mary in voluiue I of the Wisconsin survey' report. Prof. Van 
Hise has since reviewed this subject.* While he emploj^s the 
same data he has also mentioned one other. It is the non- 
conformable position of the sandstone near Agogebic lake upon 
the rocks of the Penokee series. The conglomerate here con- 
tains rolled pebbles from the Archean, from the Penokee series 
and from the Keweenawan, the last, consisting of "quartz por- 
phyry and certain phasesof basic eruptives," being considered 
proof that this sandstone here is of later date than the Ke- 
weenawan. Admitting the references of all the pebbles as 
expressed by Van Hise and their significance, there is lacking 
still that same element which has been found wanting in sev-- 
eral other similar cases, viz. : Is this conglomerate at the bot- 
tom of the series? It is evident that in a region which is 
subject to prolonged progressive subsidence the same fragmen- 
tal series may form a conglomerate in contact with varioiis 
terranes as the ocean advances. This would show non-con- 
formity on each of them. But this circumstance does not 
prove non-conformity upon the beds nearest related in time to 
the transgressing formation. In order to prove an erosion 
interval preceding such transgressing formation it is neces- 
sary to find its lowest beds to be composed of a conglomerate 
and that they lie non-conformabl}^ upon the strata that next 
preceded the disturbance in point of time. In the case of an 
eruptive age it is hardly sufficient to find simply some of the 
upper parts of a sandstone non-conformable upon some of the 
earlier parts of the same formation to warrant the assertion 
of a long land interval between those extremes. 
Further, the origin of the supposed Keweenawan pebbles is 
not so certainly established as would be desirable to prove the 
post-Keweenawan age of this conglomerate. If the situation 
l>e considered a moment it appears that the quartz-porphyry 
pebbles may have been derived from the south. The pebbles 
from the southern complex must have come from the south. 
That shows the existence of powerful drift from that direc- 
tion. According to the Wisconsin geologists there is a large 
amount of quartz-porphyxy associated with the "Archean" 
•quartzytes further south. f These probably are higher in the 
*The Penokee iron-bearing series of Michigan and Wisconsin. Mon. 
Aix, U. S. Geol. Sur. 
fGeology of Wisconsin, vol. ii, pp. 249, 520. 
