472 Mr. Austen on the 



decided effects on the physical features of the country. They usually occur either 

 at intervals along the ridges of hills, or at the points of conical elevations, in the 

 form of protruding masses ; and their injected nature is estabhshed by such natural 

 sections as that presented at the break in the line of hill through which the Lemon 

 stream flows past Holbeam Wood (PI. XLII. fig. 2, Hobbin Wood, Ord. Map, two 

 miles west of Newton Bushell) , by the alteration which they have produced in the 

 rocks which include them ; or again by such horizontal sections as the one given in 

 the woodcut, fig. 13, which represents the ground-plan of a dyke to be seen near 

 West Down. In this instance it is very evident that the intrusion of the trap was 

 subsequent to the time when the slates took their present structure. The whole line 

 of the lower Hmestone band of Bickington affords numerous instances of intrusion of 

 trappean rocks amidst slates and limestones ; in the town of Ashburton, and in 

 the street leading out to Buckland, is a large quarry in which broken beds are 

 visible completely included in the greenstone. 



Wherever the trap produced a conical slate hill (a very common feature), ra- 

 diating fractures necessarily resulted ; and wherever these have been of sufficient 

 width to allow the trappean matter to ascend to the surface, we have there a central 

 mass with radiating branches, proving the fluid state of the injected rock : instances 

 may be seen between Woodland and Bickington ; road-cuttings, and other exca- 

 vations, al&o expose the terminations of small veins which do not quite reach the 

 surface. 



The amount of alteration which the heated trap lias effected, varies considerably. At Holbeam the 

 dykes have quite obliterated the lines of deposition, and converted the slates into a compact flinty mass ; 

 at Bishopsteignton, where we may suppose the beds were originally in a slight degree calcareous, as they 

 still retain traces of organic remains, the shales were reduced nearly to fluidity, and changed into a 

 red jasper. Flexible asbestus often occurs at the junction of intrusive hornblende rocks with the slates, 

 as at Highwick. In very many points also the eruptive trap rocks of South Devon resemble the Ophites 

 of the South of France in the general outline of the country, producing lines of conical hills ; they have 

 been erupted in a fluid or semi-fluid state ; they have possessed sufficient heat to have produced important 

 changes on the rocks in contact with them ; but they seldom have flowed over the neighbouring surface. 

 Near Totness, in the quarry by the side of the Newton road (PI. XLII. fig. 4-.), is an exception ; the 

 trap is there seen resting on a dislocated surface of slate and limestone, and at one place the laminas are 

 much contorted, as if by the weight and lateral pressure of the trap : another instance occurs at Wear. 



There are two mineral products which have resulted from these hornblendic rocks. Magnesia is 

 abundant in those layers of limestone which immediately succeed the subordinate trappean beds ; and 

 from the large proportion of that earth in all hornblende rocks, we can easily account for its association 

 with limestones which were deposited immediately after the eruptions that disseminated so much trappean 

 matter in a state of subdivision at these particular places. Again, where great trap dykes have intruded 

 among limestones, and long subsequent to their formation, as at Bickington, Ashburton, Bishopsteignton, 

 Kitley, &c., the calcareous beds invariably contain magnesia. Manganese is also very abundant in South 

 Devon, and great quantities are annually raised. In the majority of instances the metal is found in 

 immediate association with masses of erupted trap, and filling the cracks and fissures which resulted 

 from its intrusion, as at Denbury, Ideford, &c. The hornblendic are the only igneous rocks which con- 



