FROM THE EDDYSTONE GROUNDS TO START POINT. 383 



" 3. Rock Fragments and Sand Grains. — Grades I., II., and III. were 

 examined and the rock fragments identified. Advantage was taken of the 

 search for foraminifera to examine tlie sand grains in grades VI. and VII., and 

 the residues from the acid solutions after dissolving out the carbonate of lime 

 have also been examined. 



"There are three clearly defined geological areas represented by these 

 samples. 



"The first, represented by a single dredging only, occurs at 90 (Ground V.), 

 to the north-east of the Eddystone and south of Rame Head. Here the 

 rock fragments are almost entirely Devonian in character, the gravel consisting 

 of fragments of slate and quartz from veins in slate, closely resembling the 

 rocks of Penlee Point. A little Trias occurs. 



"The second area covers the whole of the rest of the samples examined 

 from the Eddystone neighbourhood. This may be called the Triassic area. 

 The prevailing rock fragments are the detritus of Triassic rocks ; mottled 

 sandstones, red jaspideous sandstones, micaceous sandstones, and drab sand- 

 stones. This must be taken as an indication and not an exhaustive definition 

 of the nature of the rocks. In consequence of the presence of these rocks 

 the bottom-deposits contain a great amount of free crystalline silica and a 

 considerable amount of iron. Associated with the Trias are some Devonian 

 rocks, but the amount of these present is comparatively small, rarely rising 

 above 15 per cent, and never reaching 16 per cent. Fragments of the 

 Eddystone gneiss also occur, but in small quantity except in the immediate 

 neighbourhood of the reef. 



"The third area is represented by samples 105 and lOG (Ground XVII.) 

 from the neighbourhood of the Bolt. This may be described as the ' Archean 

 Schist ' area. The rock fragments are largely derived from mica and chlorite 

 schists, similar to or the same as those forming the promontory of the Bolts. 

 In 105, II., the Archean schists form 78-75 per cent, of the whole rock 

 fragments, Flint pebbles form 12-74 percent., and Triassic rocks 8-51 percent. 

 The Prawle Stony Ground (XVIII.) corresponds geologically to 105. 



"Further south, at the point at which 106 was dredged, the Archean falls 

 to 48-90 per cent, and the Flint rises to 40-44 per cent., apparently indicating 

 the proximity of a cretaceous area. Trias is still present as 10-66 per cent. 



"These Archean rocks are referred to in 'The Rocks of South-west 

 Devon,' R. N. Worth, Devonshire Association Transactions, 1887, as well as 

 elsewhere in the same Transactions 1)y other writers. 



" Briefiy stated the geological results are as follows : — 



" It is almost certain that the gneiss and granitoid rocks which constitute 

 the summit of the Eddystone reef are restricted in area of exposure to little 

 more than the summit and a portion of the sloping sides of the reef. It 

 is only at 87 that the gneiss preponderates over all other rock constituents 

 in the gravel, and here in 87, III., it reaches 74-07 per cent, of the 

 total rock constituents. This is on tlie immediate western fringe of the reef. 



"At 85, a little west of 87, the gneiss forms 25 per cent, of the rock 

 constituents, and at 103, south-east of the reef, it forms 26 per cent. 



