628 STUDIES FOR STUDENTS 



is a structural break between the two, the inference being that 

 one formation was folded and truncated before the overlying 

 clastic formation was deposited upon it. This occurs in the 

 Marquette district of Michigan. 



Another case is as follows : A lower shale or grit may be 

 overlain conformably by a sandstone. By cementation these 

 formations may become indurated ; the grit into graywacke, and 

 the sandstone into quartzite. After this an orogenic movement 

 may develop cross cleavage or cross fissility in the softer, lower 

 formation, the secondary structure abutting against and sharply 

 terminating at the overlying quartzite. The same movement 

 may develop a pseudo-conglomerate in the overlying formation. 

 In the later stages of the process the differential movement may 

 tear off fragments of the lower slate or schist and include them 

 with the broken, harder formation. Such a pseudo-conglomerate 

 simulates to a remarkable degree a basal conglomerate resting 

 unconformably upon an earlier series, in which it might be sup- 

 posed that the secondary structure was produced before the 

 overlying formation was deposited. Exactly these relations 

 obtain within the Ajibik quartzite formation of the Lower Mar- 

 quette series, northeast of Palmer. At first it was supposed that 

 the pseudo-conglomerate was basal and marked an unconformity, 

 and it was only after the locality was repeatedly visited and 

 studied in the utmost detail that the true relations between the 

 two formations were discovered. 



Similar phenomena may occur between formations different 

 from those above described, in which the inferior formation is 

 weaker than the superior formation. 



As another illustration of the great difficulty in sometimes 

 distinguishing between the two, a case in the Adirondacks may 

 be cited in which a thick formation of gneiss is overlain by a bed 

 of crystalline limestone containing interlaminated smaller beds 

 of gneiss. The whole series has been closely folded. The 

 gneiss, as a result of the folding, is closely corrugated, and to 

 a certain extent its upper folds are truncated by the shearing 

 action. The limestone has acted like a fluidal substance, accom- 



