ANALYTICAL ABSTRACTS. 537 



concluded that the soft sandstone belongs to a distinct and later geological 

 age than the Trap range. 



The character and origin of the copper deposits are discussed. 



Comments. — The major structural conclusions independently reached by 

 the Michigan Geological Survey are nearly identical with those which have 

 been published by the officers of the United States Geological Survey. The 

 same may be said as to the origin of the iron ores. Upon a few points there 

 is, however, a difference of opinion. 



The unconformity which exists between the Lower Marquette and the 

 Basement Complex marks a distinct geological age, whether gneissoid granites 

 composing the latter are metamorphosed eruptives or metamorphosed sedi- 

 mentary rocks. It is true that a sedimentary formation resting upon an 

 eruptive, and deriving material from it, is no evidence of a geological break 

 if the eruptive is a surface rock and has not been altered before the overlying 

 formation was deposited. If, however, the eruptive is a deep - seated rock, 

 or has been so sheared and folded as to take on a schistose structure before 

 the deposition of the succeeding formation, and has consequently reached the 

 surface by erosion, the discordance may mark as great a geological break as 

 an unconformity between a metamorphosed sedimentary rock and an unaltered 

 overlying series. 



That there is more than one geological period represented in the Cascade 

 formation seems unlikely, and in a later note by Dr. Wadsworth this idea is 

 apparently abandoned. If any gneisses of the Huron Mountain prove to be 

 unconformably upon, and to have derived material from, an older gneissoid 

 granite series, it is probable that this new series will be found to be equivalent 

 to the Lower Marquette or Upper Marquette series rather than to belong to 

 the Cascade formation. 



Jaspillite and ore are tentatively placed as one of the kinds belonging to the 

 Cascade formation or Basement Complex, although the major portion of them 

 are placed in the higher series. No large areas of this rock yet discovered 

 would be here placed by the reviewer. The jaspillite of Ishpeming and 

 Negaunee doubtfully referred to the Cascade is believed to be a sedimentary 

 deposit of the same age as similar rocks of the Lower Marquette series. 



That the jasper near the base of the iron - bearing formation at Cascade is 

 interlaminated with layers of fragmental material is not sufficient evidence 

 that the jasper is or has been derived from a mechanical sediment. The 

 inferior formation of the lower Huronian is usually, if not always, a clastic 

 deposit, resting as it does unconformably upon an earlier series of granites, 

 gneisses and schists. This fragmental formation usually grades up into the non- 

 fragmental formation of the iron - bearing member, and before continuous 

 pure non- clastic sediments are reached there are often several alternations of 

 the two kinds of deposits. Such occurrences are exactly analogous to the 



