588 A. W. GRABATJ TYPES OF SEDIMENTARY OVERLAP 



removed to the extent shown in northern Scotland. The deposits here, 

 aggregating 7,000 feet in thickness, are terriginous throughout. In 

 western England the Cambric beds have a more offshore character, con- 

 sisting of basal sandstones, followed by calcareous beds and by the Dicty- 

 onema and Shineton shales. There are some intercalated conglomerates, 

 and the thickness of the series is much less than in Wales, thereby indi- 

 cating some oscillating conditions. On the whole, however, there seems 

 to have been a steady advance of the sea westward. 



In northern Wales the lowest Cambric beds are the Llanberis slates, 

 3,000 feet thick, and the Harlech grits, a continental formation 6,000 

 feet thick. This is followed by 225 feet of Menevian, and then by 3,100 

 feet of the Lingula flags (Upper Cambric), which in turn are succeeded 

 by the Tremadoc (1,000 feet). The Llanberis slates rest upon quartz 

 felsite, and have furnished Conocoryphe and Hyolithes. They are most 

 probably to be classed as Middle Cambric, which rises the transgression 

 as of that data in north Wales. The transgression reached Anglesea, in 

 northwestern Wales, toward the close of Cambric time ; for here the pre- 

 Cambric crystallines are succeeded by basal quartz- jasper conglomerates 

 of Tremadoc age,* all the earlier beds having come to an end and being 

 overlapped by the highest of the series. The fossiliferous Tremadoc beds 

 pass upward into beds with Arenig fossils. f The basal conglomerate thus 

 rises in the scale, until from Lower Cambric in southern Wales it has 

 become uppermost Cambric in northwestern Wales. 



In Scandinavia the basal Cambric is the Fucoidal sandstone, which 

 appears to represent a reworked continental deposit, as indicated by the 

 presence of the "drei-kanter." This sandstone is probably not of the 

 same age throughout, but represents higher and higher horizons toward 

 the old land, though still holding its place as a basal bed resting directly 

 upon the crystallines.^ It represents, in other words, a basal clastic of a 

 transgressing sea. 



The well known fact that the lowest Cambric deposits of Bohemia are 

 of Middle Cambric age may be cited as another example of the overlap of 

 higher on lower formations in a continually transgressing sea. The basal 

 beds resting directly upon the pre-Cambrics are coarse conglomerates and 



* Hughes : Quart. Jour. Geol. Soc, vol. xxxvi, p. 237 ; xxxviii, p. 16. 



t Professor Hughes, in his second communication, refers the species of Orthis found in 

 the sandstones above the basal conglomerate, which he formerly identified as O. caraus- 

 sii, an Arenig or Tremadoc species, to O. McTcsii, a Menevian species, making the two 

 types con-specific. On the strength of this, he suggests the possible Menevian age of the 

 basal Cambric beds of Anglesea. Since they are, however, followed without break by 

 typical Arenig strata, and since the species of Orthis found is a typical Tremadoc and 

 Arenig species, even though considered only a variety of the Menevian O. hicJcsii, the 

 reference of these basal beds of Anglesea to the Tremadoc is probably correct. 



% See Nathorst : Sveriges Geologi, p. 145. 



