1865. j Science in the Provinces. 529 



ence of overlying beds of great thickness, which subsequently to the 

 change must have been swept away. There was again a further de- 

 struction, for the Bigbury Bay outlier of Triassic conglomerate is 

 mainly composed of fragments of the schists. The Dartmoor granites 

 have a similar history. They have been formed under the pressure 

 of overlying masses, and according to some modern views they were 

 formerly in the state of beds, which have therefore been destratified ; 

 in this case they must have been derived from previously existing 

 rocks. Denudation also took place during pauses in the process of 

 sedimentation. This is seen not only in the Triassic series, but also 

 in the Devonian limestone near Hope's Nose, Torbay. The limestone 

 is there seen to be composed of two unconformable series : the lower, 

 which includes several interstratified beds of volcanic ash, having 

 been violently contorted and denuded before the deposition of the 

 upper undisturbed beds. The Torbay limestones, however, represent 

 but a small fraction of the Devonian era. The outliers which are 

 found scattered over the county indicate the former extension over a 

 great part of it of the Trias, greensand, and chalk, which have since 

 been almost entirely removed. Thus there are outliers of Trias on 

 the shores of Bideford and Bigbury Bays, and one of greensand near 

 Bideford, forty-three miles from the principal mass of the formation 

 at Blackdown. On some of the greensand hills there are thick accu- 

 mulations of chalk flints unrounded and untravelled. The chalk 

 which formerly contained them, and which must have been of great 

 thickness, has been quietly removed by the dissolving power of rain- 

 water containing carbonic acid. 



The islets fringing the Devonshire coasts are outlying relics of an 

 ancient coast-line. Other proofs of destructive agency are afforded by 

 " terraces of denudation," where the retreating tide leaves a rocky 

 platform instead of a sand or shingle beach ; the water having reduced 

 to a plane surface the outcrop of highly-inclined strata. Similar 

 terraces are seen on the summits of limestone hills now inland. 

 Raised beaches occur round the coasts about 30 feet above the sea, 

 usually occupying platforms or terraces in hard rocks. Occasionally 

 slabs of rock are seen to be cemented to the faces of limestone cliffs. 

 These have formerly fallen into fissures, and been there cemented by 

 water containing carbonate ■ of lime. Subsequently the mass of rock 

 lying between the fissure and the sea has been removed by the waves, 

 leaving the blocks in their present position. The Budleigh Salterton 

 beach pebbles offer striking proofs of repeated denudation. The 

 beach extending from Sidmouth to the eastward extremity of the 

 county is almost entirely composed of quartz pebbles derived from a 

 pebble bed near Budleigh Salterton. The pebbles have been re- 

 cently shown to contain Lower Silurian fossils similar to those occur- 

 ing in the quartzite beds of Normandy (see ' Quart. Jour. Geol. 

 Soc.,' vol. xx. p. 283 ; Aug. 1864). Mr. Pengelly hence deduces the 

 following series of changes : — 1st. The denudation during the Lower 

 Silurian era of pre-Silurian rocks containing quartz, and the depo- 

 sition of the triturated material. 2nd. The production, by denudation, 

 of a large amount of debris, which was deposited on the arenaceous 



VOL. II. 2 o 



