198 TEANSACTIONS OF THE [1892 



to Gwynn's falls. A few patches of this qnartzitic mica 

 schist still remain in the soil of gardens and lots beyond 

 Mount Clare Station and on Peabodj Heights. 



Summary of Ohservations. 



The literature describing the various details upon which 

 later investigations have proceeded towards a theory of 

 the origin and development of the sedimentary series 

 resting between the crystalline rocks and the base of the 

 Greensand Cretaceous, or true marine, deposits may be 

 found in the publications of H. D. Rogers, H. B. Rogers, 

 T. A. Conrad, P. T. Tyson, J. S. Newberry, C. A. White, 

 W. M. Fontaine, W. J. McGee, N. H. Darton, and the 

 present writer. 



The two writers named last adhere to the theory that 

 there is but a single unit of geological structure between 

 the top of the Triassic and the base of the Greensand Cre- 

 taceous, or Severn formation of Darton ; and to that unit 

 they persistently give the name Potomac formation. 



Our own observations, as given above and elsewhere, 

 do not coincide with that view, but point to the conclu- 

 sion that this complex of beds constitutes at least three 

 structural units, separated by distinct time-breaks which 

 mark periods of transition from one series to the next. 



My own belief, as based upon observations in all parts 

 of the field, is that the assemblage of beds cited above is 

 the remnant of a great fresh or slightly brackish water 

 " Period,'''' which was built upon the border of the crys- 

 talline continent before the salt-water ocean had become 

 established as a fundamental portion of the territory. 



This Period is divided into three natural sections by 

 stages of erosive and transportive activity, which greatly 

 altered and partly rearranged the previously deposited 

 sand beds. The basal member, or Potomac formation 

 proper, was laid down upon the broken and hollowed- 



