212 J. B. Bill— The Palceozoics of Wed Cormcall. 



greenstone is in close proximity to Radiolarian cherts, but the 

 evidence is insufficient to decide whether it is intrusive or con- 

 temporaneous. 



The Devonian base on the Gramponnd and Prohiis horizons. 



The evidence obtained from the Manaccan area, v^^here the Upper 

 Palaeozoic rocks occur as a great outlier amongst the Lower Palaeozoic 

 group, has thrown light on the nature of the junction further to the 

 north that marks the main boundary between those formations. 

 The fine-grained conglomerate of Grarapound and Probus was 

 recognized in 1902 as the equivalent of the conglomerate of the 

 Manaccan Series. This inference was amply confirmed by micro- 

 scopical determination undertaken by Dr. Flett. It contains 

 a similar assemblage of rock fragments, i*esembling one another 

 so closely as practically to preclude the possibility of their derivation 

 from different sources, or from the same source at widely different 

 periods. It was at the same time recognized that it might represent 

 an unconformity between the Silurian and Devonian rocks of 

 Cornwall, an inference which the recent detailed investigation of the 

 Manaccan area has shown to be correct. This fine-grained con- 

 glomerate is associated with slightly calcareous sandstones and clay 

 slates, and the whole assemblage is precisely equivalent to that 

 represented by the Manaccan Series, except that in the latter, besides 

 the fine-grained conglomerates like those of Probus and Grampound, 

 these beds frequently present an extreme coarseness, many of the 

 included boulders exceeding a foot in size. As already remarked, the 

 conglomerate of that area is by no means continuous, but the base 

 of the formation is frequently of fine texture. There is, moreover, 

 a tendency for the conglomerate to die out in a westerly dii'ection, 

 so that the demarcation of the Manaccan Series from the older 

 Palseozoics is attended with difficulty in the cultivated areas. These 

 conditions are exactly reproduced in the Probus horizon. The coarse 

 conglomerates, analogous to those of Manaccan, occurring only in 

 the district further to the east, where they have been mapped by 

 Mr. Eeid in the Gorran Sheet (353), while in the western direction 

 towards the Bristol Channel even the fine-grained conglomerates of 

 Probus and Grampound disappear. The rock series of Probus 

 likewise corresponds in strike with that of Manaccan, having 

 a general east and west trend and oblique to that of the adjoining 

 Lower Palaeozoic strata. The southern boundary extends from 

 Porth Towan on the west to Probus on the east, whence it has been 

 traced by Mr. Reid to the north of Tregoney, where it sweeps to the 

 south, and thereafter follows an irregular course to the coast at 

 St. Michael Caerhayes. 



The Grampound and Probus Beds have hitherto yielded no organic 

 remains except at Ladock, where an indeterminable brachiopod has 

 recently been discovered. To the north, however, the work of 

 Messrs. Reid and Scrivenor shows that they link on with Lower 

 Devonian fossiliferous horizons in the Newquay district. They, 

 moreover, have close affinities to the Devonian, not only in their 



