Xll INTRODUCTION. 



Red Sandstone beds of Devonian age are associated with the slates on 

 botli sides of Plymouth Sound, and in some other places on or near the 

 coast. 



Devonian Limestone. — The Limestone of the Plymouth district "forms 

 a band half-a-mile in width and nearly six and a half miles in length. Its 

 main western extremity is in Devonport Dockyard, but there are detached 

 beds on the other side of the Taraar. It breaks off on the east about 

 Sherford. Its general elevation is much lower than that of the slate hills, 

 averagmg about a hundred feet ; and whilst its summit-lme has been 

 tiiily described as being level as a wall, its continuity is broken at several 

 points." A little south of Hay Farm it attains 180 feet, and at Rounds 

 Nest, south of Yealmpton village, 150 feet. " The Tamar, after rounding 

 its western extremity, passes through it at Cremyll ; the Plym has a 

 narrow channel at Cattedown ; and at Stonehouse Pool, Millbay, and 

 Sutton Pool, the waters of the Sound find access through the barriers to 

 basins worn out of the slate rocks behind." (pp. 459-60.) About three 

 miles east of the extremity of the main mass considerable beds appear at 

 Kitley and Yealmpton ; and about two miles beyond there is a narrow 

 band, about one mile long, running in a south-east direction from Sequers 

 Bridge towards Modbury. 



" The limestone varies much in colour and structure. Both on the north 

 and south it graduates into the slate through calcareous shale. It abounds 

 in fossils, chiefly coralline in its more massive portions, whilst some of 

 the exterior beds have yielded large quantities of bivalves and univalves ; 

 and others, with the adjoinuig slates, are remarkably fruitful in crinoidal 

 remams. In texture it is generally highly crystalline, and in colour very 

 various, ranging from black, through red, yellow, dove, and gray, to 

 white." (p. 460.) The Yealmpton limestone is largely doloriiitic, but 

 magnesian luuestone is of general occurrence, associated with the ordmary 

 forms. 



Elvaiis traverse Morwell Down, the southern of which extends nearly 

 to Horrabridge ; one crosses Roborough Down in an east and west 

 du'ection ; another occurs near Roborough village, and a porphyiitic one 

 crosses the country from the valley of the Plym to near Knackersknowle.* 

 There are numerous other bands and patches of rock which approach the 

 character of elvans, but it is difficult in several cases to decide precisely 

 as to their character. In like manner many of the rocks conunonly 

 classed as trappean are undoubtedly merely altered rocks of sedimentary 

 origin. The genuine trappean rocks where decomposed yield some of the 

 richest soil in the district. 



Granite. — Granite rocks, forming a part of the south-western portion 

 of Dartmoor, occupy the north-east of the area. A detached mass forms 

 the emmence of Ilemerdon Ball and part of Crownhill Down. In the 



* De la Beche, Rep. 184. 



