1854.] HORNER IGNEOUS ROCKS IN CAWSAND BAY. 363 



rock of a purple colour, and containing some hard nodular concre- 

 tions which effervesce slightly with acid * ; and in the vertical cliflF 

 near this spot, there is an interruption to the continuity of the ordi- 

 nary stratification hy a mass so much resembling a dyke that I 

 expected to find it to be porphyry, but it proved to be a schistose 

 rock resembling that on the shore above, mentioned as being inter- 

 stratified with the true slate f. It has, however, a cleavage fracture at 

 a considerable angle to the plane of stratification of the adjoining slate, 

 and which does not extend into the slate. 



The Rev. D. Williams read a paper to the Geological Section of 

 the Meeting of the British Association at Plymouth, in 1841 X) in 

 which he refers to the part of the coast now under consideration, 

 stating that " from Redding Point to the great mass of porphyry 

 near the fishing houses, there is one uninterrupted series of varieties 

 of volcanic ash, sometimes passing into clay-slate, interstratified 

 among the thick red sandstone beds seen in the east and west clifFs 

 of the Sound." The schistose rock above described may possibly be 

 what Mr. Williams calls Volcanic Ash, but the term would be alto- 

 gether inapplicable to the far greater proportion of the rocks in this 

 locality §. 



Farther east the slates and associated arenaceous beds are in a very 

 disturbed state, both as to direction and dip ; highly inclined beds, 

 dipping S.S.W., abut against others that are vertical, and these last 

 bend round into a stratification inclined not more than 10° and 

 toward N N.W.|| I tried, but without success, to make a correct 

 drawing of this singular and not very intelligible disturbance ; the 

 following rude sketch will, however, give a better idea of it than mere 

 words can do. 



Diagram of the disturbed Slates in Cawsand Bay, near Plymouth. 



The nearly horizontal beds are very hard, have greenish-grey spots 

 and stripes, and divide into sharp angular blocks, both sides of the 



* Specimens Nos. 15, 16, & 17. f Specimen No. 18. 



X Report for 1841, Transactions of the Sections, p. 62. 



§ Mr. Williams, in the same paper, goes so far as to say that in his opinion the 

 killas or clay-slates of Devon and Cornwall are volcanic products in a stratified 

 condition. 



II Sir Henry De la Beche evidently refers to this part of the cliff, and ascribes 

 the disturbance of the stratification to a fault. 



