202 STEATIGEAPHICAL GEOLOGY 



Lower Devonian. The recent work of the Geological 

 Survey in Cornwall has shown that the basal boundary of the 

 Lower Devonian crosses that county along an east and west line 

 from Forth Towan on the west to near Tregony on the east, 

 whence it bends southward to Veryan and Nare Head (see Index 

 map, 4 miles to 1 inch, sheet 25). From this line northward to St. 

 Evan, Bodmin, and Plymouth, the country is occupied by Lower 

 Devonian grits and slates, interrupted only by the intrusive mass 

 of granite which lies to the north of St. Austell and St. Blazey. 



Along the line of boundary above mentioned Devonian grits 

 and conglomerates rest unconformably on rocks which are either of 

 Ordovician or Cambrian age ; but these grits do not appear to be 

 the oldest Devonian strata. The surveyors think that the Dart- 

 mouth slates are concealed beneath an overlap of the higher beds 

 on to a sloping shore-line, and believe the conglomerates of 

 Grampound, Probus, and Nare Head to be shore deposits belonging 

 to the Meadfoot division of the series. The general structure of 

 North Cornwall is expressed in Fig. 65, which indicates the 

 supposed overlap and the series of monoclinal flexures by which 

 the beds seem to be repeated, but the section must be regarded 

 as merely a diagram of the theoretic structure of the area. 



The Dartmouth slates form a thick series of purple, red, and 

 green slates with frequent bands of hard and fine quartzitic grit 

 such as are called quartzo-phyllades in France and Belgium. They 

 seem to have a thickness of over 2000 feet, but the base has not 

 been seen because it is not brought up in any of the anticlinal 

 flexures. The general position of these beds, according to the 

 view taken by the Geological Survey, is indicated in Fig. 65. 



The only definite marine invertebrate yet found in this series is 

 Bellerophon trilobatus, which is stated to be fairly common though 

 badly preserved. Fish remains also occur belonging to the genera 

 Pteraspis, Cephalaspis, and Parexus. 



The Meadfoot Group is another set of slates and fine grits, but 

 is distinguished from the Dartmouth slates by the colour of the 

 beds, this being always dark grey. In the Looe district the 

 lowest beds are hard grits and quartzites with bands of grey slate, 

 and it is these beds which are believed to pass southward into the 

 coarse grits of Grampound and Probus. The faults and plications 

 are so numerous that the thickness may be anything from 500 to 

 1000 feet. The Meadfoot Beds are more fossiliferous than those 

 below, but the specimens are seldom in good preservation. The 

 following are some of those which have been determined : Pleuro- 

 dictyum problematicum, Pachypora ( ? cristata), Spirifer primcevus, 

 Sp. hystericus, Orthotetes hipparionyw, Stropheodonta giyas, Rensellceria 



