74 
IOWA ACADEMY OP SCIENCES. 
vertible, based as it is on both detailed field evidence and 
microscopic examination of the rocks, states that in the area he 
studied the stratified rocks within the basin are the oldest rocks, 
the granites surrounding the basin are next in age, then come 
the diorite, diabase and melaphyre in order. He also concludes 
that the granites, felsites, diorite, diabase and melaphyre are all 
eruptive rocks, not derived by metamorphism from any part of 
the stratified rocks. 
These conclusions relate to the part of the basin north of 
Boston where evidence is most abundant and complete. In the 
fall of 1894, it was the writer’s privilege to study the south- 
western part of this basin and to prepare the accompanying 
map, the plate of which is now kindly loaned by the Boston 
Society of Natural History. This map and the paper that 
originally accompanied it* give the location of outcrops to be 
found in the area under consideration and a discussion of the 
relation of those outcrops based in part on the field evidence 
and in part on the microscopical character of the rock. The 
basin itself was found to extend in narrow areas farther south- 
west than formerly supposed. 
* “On the Southwestern Part of the Boston Basin,” Proc. Boston Soc. Nat. Hist. 
Vol. XXVI, June 28, 1895. 
