222 
IOWA ACADEMY OF SCIENCES. 
is considered as opposed to the idea that the formation is of that 
age; and it is referred to the Huronian. 
Recently Cnlver§ reported the discovery of a large trap-like 
dike in the midst of the Sionx quartzite in Brandon township, 
Minnehaha county, South Dakota. Thin sections were exam- 
ined by Hobbs II and pronounced to be a well-defined olivine 
bearing diabase. 
A few months later, in a summary of what is known of the 
geological formations of lowal, attention was called to the 
finding of quartz-porphyry in the sinking of deep wells in the 
northwestern part of the state as indicating very old eruptive 
rocks at no very great distance beneath the present surface. 
In bringing together the various views regarding the age of 
the Sioux formation, it was shown that the consensus of opinion 
led to the inference that it was pre- Cambrian. 
Subsequently* the existence of eruptive rocks in the quartz- 
ite area was discussed at length and the conclusions reached by 
others in regard to geological age stated. 
Since the remarks last mentioned were published the Sioux 
quartzite has been visited personally and many of the principal 
outcrops carefully examined. 
Without going into details of the various observations, it 
may be stated that impressions were seen at several points in 
the bedding planes of the quartzite which resembled so much 
those of lamellibranchs of the Cardium and Cytherea types 
that it must be confessed that notwithstanding strong precon- 
ceived notions regarding the great antiquity of the Sioux rock, 
faith in its very old age was considerably shaken. The impres- 
sions were in such large slabs of quartzite that it was impossible 
at the time to obtain more than a single imperfect one; it was 
the intention to revisit the region and search carefully for addi- 
tional and better material, but the opportunity did not again 
present itself. Hayden’s reference to abundant casts in the 
argillaceous portion of the formation in another part of the 
region was recrJled. Regarding the age of the Sioux formation, 
it may be said that while it should be considered as pre- Cam- 
brian age — until indisputable evidence is produced to the con- 
trary — there exists now a certain element of doubt concerning 
the accuracy of this view. 
§ Trans. Wis. Acad. Sci., Vol. VIII, p. 208. Madison, 1891. 
II Ibid. 
1 Keyes : Iowa Geol. Sur , Vol. I, pp. 15-19. Des Moines, 1892. 
Keyes: Proc. Iowa Acad. Sci., Vol. I, part 3, p. 21. Des Moines, 1893. 
