390 Scientific Proceedings, Royal Pubtin Society. 



considerable. For the extreme alkaline range, pH 9 4 to 10*0 thymol 

 phthalein is quite the best indicator, though it fades quickly. 



In the district round Plymouth, in the counties of Devon and Cornwall, 

 the oldest Devonian formations are on the coast, and the newer are 

 encountered as one proceeds inland in a northerly direction. There are, 

 however, numerous isolated patches of rocks of igneous or plutonic origin. 



Thus in Cornwall, near Eame Head, to the west of Plymouth Sound, are 

 found the Dartmouth Slates, in the Lower Devonian. They also extend 

 across to the eastern side. The soil is a dark chocolate brown, and gave the 

 reactions pH 6"7~7'3, as shown in Table I. This siliceous rock yields a soil 

 which may be caused to become acid or alkaline ; it appears to have but little 

 buffer action. Some of the results for fields may be due to the addition of 

 limestone as a manure; those parts near the coast — and none are far fiom it — 

 may be enriched and neutralized by the shells of mussels, periwinkles, and 

 such like brought up by birds, by the use of sea-weed as manure, and by the 

 strong southerly winds carrying minute amounts of sea-spray in suspension. 

 These factors all appear to lessen the production of strongly acid soils near 

 the sea, and they are by no means negligible in their action over prolonged 

 periods. Inland from the Dartmouth Slates come the Staddon Grits, which 

 consist of reddish grits and some slates and shales, with thin limestone 

 bands. 



This formation runs east and west on both sides of Plymouth Sound, as 

 shown in the maps of the Geological Survey. The rocks and soil derived 

 therefrom are of a deep red colour, and stand in marked contrast to the 

 Dartmouth Slates, the soil from which is dark brown. In the table may be 

 seen the results given by this formation. It appears to be of definite, but 

 low, alkalinity, except under special conditions, such as where pine needles 

 make up most of the superficial soil, which may be as acid as pH 5'2, and on 

 slate. Apparently these coarse sandstones have become permeated with 

 calcium carbonate from the limestones which originally rested over them. 

 Whatever the cause, the soil is but liltle less alkaline than that lying directly 

 on limestone, provided it is not practically disintegrated limestone. 



The line of separation of the Dartmouth Slates and Staddon Grits lies in 

 the bay containing the Cawsand and Kingsand village. Just to the north of 

 this, on the sound, and for about a quarter of a luile inland, lies a patch of 

 felsite. On this the soil is blackish, and everywhere acid, save in a small 

 glen, bounded on two sides by Staddon Grit, where drainage water and 

 downward movement of soil have resulted in a sufficient admixture of calcium 

 salts to neutralize the acidity. This appears a fertile spot, and is divided 

 up into allotments. 



