KOCKS OP SOUTH DEVON. 491 



local successions are not capable of general application. Thrust-faults 

 on the axes of inversions are frequently present on a small scale ; 

 but to ascertain their existence on a large scale demands palaeon- 

 tological data of a most definite character, which from the com- 

 parative rarity of characteristic fossils does not appear to be 

 obtainable. AYe have therefore to fall back upon such information 

 as can be procured of the general types of Upper-, Middle-, and 

 Lower-Devonian faunas ; for without these an appeal to stratigraphy 

 can only lead the enquirer to a mass of apparent^ contradictory 

 details. 



From this it must not be inferred that the occurrence of indi- 

 vidual characteristic forms is anywhere of weight unless supported 

 by sufficient positive, negative, or comparative evidence, or that its 

 admission in any way violates definite stratigra])hical position. 



Although the lithological constituents of the Upper-, Middle-, and 

 Lower-Devonian beds are broadly distinguishable, j^et there is no 

 definite lithological boundary between the groups. The Lower 

 Devonian is mainly distinguished by the occurrence of sandstone 

 and grit ; but the upper beds are slates or shales passing, where 

 unfaulted, indistinguishably upward into the Middle Devonian 

 slates. Li no part of the Lower-Devonian area have igneous rocks 

 been found. 



The Middle Devonian consists of limestones upon shaly limestones 

 upon slates. 



The slaty and shaly basement-limestone and underlying slates 

 represent the Calceolen-Kalk and Calceolen-Schiefer. The Calceolen- 

 Schiefer is somewhat variable in character, the irregular grey slates 

 with fossiliferous len tides, developed in Berry Park, and the even 

 grey and bluish-grey slates of Mudstone Bay, being different types. 

 These slates and the overlying thin limestones are characterized by 

 the occurrence of Spirifer speciostis. 



The Middle-Devonian limestones are well bedded in the lower 

 part of the mass, where they *are usualty dark-grey, -and contain 

 Heliolites porosus, C ijstijohyllum vesiculosum, CyatJiophyllum Jielian- 

 thoides, and 0. damnoniense. Stringooephalus is found here and 

 there throughout the district in the bedded limestones. The upper 

 part of the Middle Devonian generally consists of pale bluish-grey 

 and grey limestone, often massive, as in the Lummaton and Barton 

 quarries, where it is composed of corals, amongst which Smithia 

 Hennahi and S. Pengellyi are noticeable. These massive limestones 

 denote the passage from the Middle Devonian, in which the Lum- 

 maton fauna occurs, to the Guhoides-\iGdi^ of the Upper Devonian ; 

 and, as it is not possible to draw any definite boundary in them, 

 they must also be regarded as partly of Upper-Devonian age. 



The Upper Devonian first becomes distinctly recognizable by thin- 

 bedded limestones and limestone lenticles intercalated with chocolate- 

 red, lilac-weathered slates and mudstones. These limestones are, 

 as a rule, red or liver-coloured and of very compact texture ; they 

 are often knubbly or concretionary, as at Lower Dunscombe, at Old- 

 chard,, near Whiteway Farm (where they contain Goaiatites), and 



