666 REPORT — 1903. 



Both of tliese observers agree in regarding certain breccias occurring at the 

 mouths of the rivers Otter and Sid as the base of the Keuper, the former of 

 which the author accepts. The latter is explained as being the same breccias 

 again brought up by the fault at the Chit Roclf. 



The writer regards the Sid section as on a far higher horizon than the 

 Otterton one for the following reasons : — The Otterton breccias are overlaid by a 

 great thickness of sandstones seen between Otterton Point, Ladrum Bay, the base 

 of High Peak Hill, and even further east. 



The fault at the Chit Hock only brings up the upper portion of these sand- 

 stones, the highest portions of which are continued to the east side of the Sid, 



The alleged river Sid bi'eccias here have no occurrence ; they are only tlio 

 uppermost beds of the red nobbly or concretionary-like sandstones which are 

 almost immediately overlaid by the Keuper marls. 



Between these alleged Sid breccias and the Otterton breccias there should 

 intervene about 150 feet or more of the mottled or current-bedded sandstones seen 

 in the localities alreadv referred to. 



TUESDAY, SEPTEMBER 15. 

 The following Papers were read : — 



1. On the Disturbance of Jtmction Beds from Differential Shrinkage and 

 similar Local Causes durinr/ Consolidation. By G. W. Lamplugh, 

 F.G.S. 



Upon returning to the investigation of comparatively undisturbed Mesozoic 

 strata after having studied distortion-structures produced by earth-movement in 

 Older Palteozoic rocks, the author's attention has been frequently arrested by local 

 disturbances of the original bedding which cannot be assigned to the agency of 

 deep-seated earth-movementi but are clearlj' due to minor stresses arising from 

 some local cause in tracts limited in extent both horizontally and vertically. 



These disturbances are most noticeable where thin bands of one kind of 

 material are imbedded in thick deposits of another kind, and along the junctions 

 where thick masses of ditt'erent lithological character occur in stratigraphical 

 sequence. 



p]xamples of the first-mentioned condition are abundant in the Hastings beds 

 of the AVealden formation, where thiu layers of clay or shale interbedded with 

 thick sands and sandstones are often disrupted into irregular patches and partly 

 mixed with the enclosing sands. The second condition is frequently illustrated in 

 junctions of the Lower Greensand with underlying clays, where strips have been 

 torn from the irregular surface of the clay and dragged up for a few inches into 

 the sands, as was seen in the recently widened railway cutting at lledhill and in 

 the pit-sections at the Dover Colliery. Similar eli'ects have often been supposed 

 to denote the breaking-up of the surface below the junction by erosive agencies, 

 but this explanation is rarely adequate. 



"While some of these local disturbances may have boen caused by unequal 

 loading within limited basins of sedimentation, in the manner discussed by 

 E. Reyer,^ the author is of opinion that in most cases the}' may be assigned to 

 local stresses resulting in part from the differential contraction of sediments of 

 diverse composition while losing their water of sedimentation, and in part from 

 their unequal yielding under equal superincumbent load. Masses of peat, sand, 

 clay and calcareous sediments accumulated under normal conditions must pass 

 from the wet state to the consolidated or partly consolidated state with different 

 time-rates and with different physical results; and we may expect to find signs of 

 local tension and readjustment along the boundaries of such masses. 



In thick wedges of strata which thin out rapidly, as, for example, in the 



• K. K. Geol. Beichsanstalt Wien ; Jahrhueh, xxxi, 1881, pp. 4.31-144. 



