Ulrich—Paleozoic Systems in Wisconsin, 
95 
we can not escape the conviction that complete emergence of the 
area embraced in the present continent occurred at many times dur¬ 
ing the almost unbelievably long course of geologic history. 
Though, as said, the evidence on which we base our conclusions 
respecting the frequency and relative durance of periods when 
the sea was withdrawn and land prevailed consists mainly of 
phenomena indicating alternating retreat and advance of the 
strandline and consequent absence of deposits found elsewhere and 
overlapping structure of those present, it is yet true that even in 
Wisconsin the sedimentary record is not wholly devoid of such 
more convincing criteria of preceding land conditions as basal 
conglomerates. These conglomerates vary greatly in composition 
of material and character of the enclosed pebbles. Naturally the 
character of both depends entirely upon the material available and 
relative nearness to the shore. Obviously, under usual conditions 
pebbles of large size can be distributed over the bottom of the new 
sea only for a mile or less out from the shore. Here and there a 
river may transport them for longer distances. The latter con¬ 
dition is suggested in some instances in Wisconsin; and in every one 
of these instances the pebbles consist of rolled quartz or quartzite 
derived from distant exposures of pre-Cambrian rocks. Moreover, 
their distribution is decidedly local. 
Under more usual conditions the basal conglomerate consists of 
material washed out of the subaerially decomposed top of the under¬ 
lying formation. If the latter consists of sand then the average 
size of the grains of quartz is larger than in the undisturbed part 
of the contributing formation. If the residual mantle of the re¬ 
submerged area comprises plates, blocks or concretionary masses 
of respectively limy sandstone, limestone or dolomite, or chert, then 
the character of the basal conglomerate, or perhaps more properly 
the initial deposit, of the succeeding formation is modified accord¬ 
ingly. Nearly always the initial deposit includes laminae or may 
be quite unstratified fine material transported by temporary sus¬ 
pension in the invading waters. 
However, in many places the initial deposit contains nothing 
having any connection with the weathered residual mantle of a 
preceding land age. An excellent illustration of this condition is 
found in the large limestone quarry at Darlington. Here there is 
unquestionable evidence of a land stage that broke the continuity 
of Black River limestone deposition. The break was long enough 
