304 S. WEIDMAN 



Value of residual clays as evidence of unconformity . — True basal 

 conglomerates have been quite generally used for many years by 

 geologists as evidence of unconformity. The occurrence of 

 residual clays or soils as evidence of unconformity has, so far as 

 known to the writer, been used only in connection with the 

 determining of the stratigraphy of the various Pleistocene depos- 

 its. If the explanation of the origin and age of the thick resid- 

 ual clays here described be the true one, and if old soils can be 

 used as evidence of unconformity between the comparatively 

 recent deposits of the Pleistocene series, then it seems to the 

 writer that residual clays and soils between other unconformable 

 series should occur, and that the presence of such residual depos- 

 its may be used as important criteria for establishing the uncon- 

 formity of rock formations throughout other parts of the geo- 

 logical column. As formations indicating unconformity, basal 

 conglomerates would belong to the first formation of the upper 

 series, whereas the residual clays would be closely related to the 

 lower series, but originating during a part or the whole of the hiatus 

 between the deposition of the lower and the upper series. Basal 

 conglomerates and residual clays would not likely be found in con- 

 tact, but the former would be expected to be found covering hilly 

 land surfaces and rocky coast lines, whereas the latter would be 

 found on gentler slopes, where the forces of erosion were relatively 

 inactive or nil. The fairly uniform and widespread occurrence of 

 thick residual clays might well be considered as strong evidence 

 of the base-leveled condition of the land floor upon which the 

 overlying series was deposited ; but isolated occurrences of such 

 residual deposits should only be considered as an evidence of 

 unconformity, for such isolated deposits are found at the present 

 day in process of development in places protected from erosion, 

 at all elevations above the sea. The full meaning of weathered 

 zones in the stratigraphic column can therefore be understood 

 only when considered with the associated geological phenomena. 



The peneplain made by sub-aerial erosion. — Peneplains may be 

 cut out of land areas by sub-aerial erosion — the work of rains and 

 streams; by marine erosion — the work of sea-waves beating 

 against a coast line; and by a combination of these processes. 



