A HISTORY OF DEVONSHIRE 



parts of the same district. Slates invariably form the lower beds ; but 

 in the area between Ashburton, Chudleigh and Brixham they are over- 

 lain by limestones mostly of coralline growth, interrupted here and 

 there at various stages by incui'sions of volcanic materials ; in the area 

 south of Totnes volcanic eruptions put a stop to their extension. At 

 Plymouth and Yealmpton limestones and volcanic rocks are also present 

 on the slates, and between these limestone areas the general horizon of 

 the limestones is marked by a mass of volcanic rocks in slates. Argil- 

 laceous slates form the bulk of the Upper Devonian, they also contain 

 volcanic rocks in places, and, although they locally furnish fossils and in 

 the limestone areas contain calcareous beds of marked character, their 

 boundary with the Middle Devonian slates can seldom be ascertained. 



Owing to the characters in common shared by different horizons, 

 and to the variations in composition, or association, displayed by the 

 same horizons traced along their strike, the Devonian subdivisions though 

 generally distinct seldom admit of precise delimitation. The whole 

 series has moreover been thrown into a number of small folds broken by 

 thrusts. 



Where rocks of different resisting power have been folded to- 

 gether, such as grits and slates, or limestones and slates, numerous faults 

 have dislocated and displaced them. 



Cleavage throughout the area is dependent on the relative powers 

 of resistance of the rocks and on the nature of the local stresses to 

 which they were subjected, consequently the same beds may be shales 

 in One place and slates in another. 



Lower Devonian. — The Lower Devonian rocks occupy the area be- 

 tween the highly altered rocks of the Start and Bolt districts and a line 

 of outcrop extending from Staddon Heights (near Plymouth) to the 

 vicinity of Sharkham Point (near Brixham), along which they rise in 

 dominant features above the Middle Devonian districts. Further north 

 they reappear in an anticline broken by faults which terminates some 

 miles west of Paignton. Another anticline still more broken by faults 

 forms the highest land in the Torquay promontory. 



Coast Sections. — The relations of the Lower Devonian subdivisions 

 are best shown by a comparison of the coast sections. The Plymouth 

 coast section under Staddon Heights exhibits a series of red-brown grits, 

 slates, shales and sandstones (the Staddon grits) in inverted and plicated 

 junction with Middle Devonian slates. The Staddon grits pass down 

 into the Meadfoot beds at Bovisand Bay, where grey slates with hard 

 grit beds and irregular knubbly, partly calcareous, slates overlie dark grey 

 slates and shales with one or two thin seams of igneous rock. These 

 beds contain occasional crinoids, Zaphrentis, etc., and at the south end of 

 Crownhill Bay give place to a slaty calcareous series with limestone 

 bands containing Monticuliporoid corals, sandy decomposed red rocks with 

 indeterminable brachiopods in abundance, and hard red grits, correspond- 

 ing to the Looe rocks. These near Andurn Point are in faulted junction 

 with glossy red and greenish Dartmouth slates. 



