74 IOWA ACADEMY OF 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 ia 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. 



* "Oq the Southwestern Part of the Boston Basin," Proc. Boston Soc. Nat. Hist, 

 Vol. XXVI, June 28, 1895. 



