28 
Records of the Geological Survey of India. 
[vol. i. 
and south, between which and the sea is a wide plain covered to a great depth with cotton 
soil, alluvium and detrital accumulations, and forming almost the whole of the Surat district. 
Although the superficial deposits are very thick, the underlying rock occasionally approaches 
and appears at the surface of the plain, and where it begins to rise almost imperceptibly 
towards the hills. It is not in all cases found to consist of trap, hut in the northern part 
of the district in the Taptee river and other places, a series of eonglomeritic, calcareous, 
arenaceous and argillaceous, rocks are found dipping at a very low angle westwards, and in 
some places containing a profusion of nummulites and univalve shells : in others further up 
in the series they contain large bones, teeth of shark-lilce fish and vegetable remains as well 
as other fossils. These rocks have been provisionally termed the nummulitic scries, and from 
their very low and sometimes undulating angles of dip, their soft nature and then- present 
position, it seems more than probable that the forces of denudation which must have acted 
with great power over the whole country reduced the surface of the portion formed of them 
below that of the harder trappean hills, wearing down these overlying rocks so as to form the 
basement of the plain. Thus wo should expect to find the nummulitic series wherever the 
rock becomes visible in the plains. And this is generally the case; however, large portions 
of the low ground are so deeply covered by the surface deposits that the rocks beneath are 
entirely concealed, and as the well known readiness with which trappean rocks yield to the 
disintegrating action of the atmosphere, may not have differed greatly from that with 
which the overlying series did so, particularly when, as here, the stratification of both dips 
in a similar direction at very low angles, it is possible that the line of demarcation between 
the two formations may he so slightly defined that the place of junction forms no stronger 
feature in the ground than is traceable here, and is easily concealed by overlying detrital 
accumulations, although further from the junction where either the trap or the nummulitic 
scries occupies the whole country en masse the characteristic shapes of the ground forming 
hills in one case, or plains in the other, become very apparent. 
The formations which occur are— 
t> , C Cotton soil. 
oen (_ Alluvium and river beds. 
Tertiary Nummulitic series. 
P Trap. 
Trap .—The lowest of these formations in geological order is the trap which occupies 
the eastern side of the district, extending into it from the hilly country to the east, nearly 
as far westwards as Turkeesaur in the north. Its boundary is concealed by the alluvium 
of the plains, hut it would appear to strike south by west so as to come out upon the sea 
shore near Bulsaar. It forms part, of the great trappean group of Central India, and the 
Western Ghats, and precisely as in those precipitous and highly picturesque mountains, it is 
everywhere found to form part of a regularly stratified series* intersected by numerous 
dykes of very similar material which arc frequently porphyritie. 
The trap beds or flows, although all very similar, consist of considerable varieties, 
ranging from solid basaltic trap to soft shaly-lookiug amygdaloid, the variously sized 
cavities of which are filled with zeolites of different kinds, and sometimes by transparent or 
amethystine quartz. Beds which are locally highly ferruginous are of common occurrence, 
and in many instances these have a red colour, and weather rapidly away into a rusty soil, 
hut in others the action of the atmosphere appears to have hardened them into a variety of 
laterite. It is sometimes observed that the upper surface of a bed only lias the deep red 
color as if an alteration had been caused by the overflowing trap resting next upon it. 
Concretionary structure is very common among these traps, none within this district 
were observed to ho oolumar, if we except a lateritic mass, which will ho alluded to further 
on. No regular order of arrangement seems to obtain among the traps, the different kinds 
* This enormous accumulation of trappean rocks, whether we consider its wide superficial extent or its great 
thickness, which in the neighbourhood of the (diets must exceed 3,000 feet, exclusive of the unknown upper portion 
removed by denudation, rnc.v well excite our astonishment. It is perhaps the largest group of stratified trappean 
materials in the world, and the vents through which these found their way to the surface have never been discovered. 
The dykes, although in some places numerous, are very Insignificant both in number and quantity compared with 
the ivst of the group, and seem (utile inadequate to having alfnrdcd exil for the bedded traps, whose regnlar lines of 
stratification may lie traced by the eye for many utiles ranging terrace-like along the sides of the Ghats’ mountains 
with a parallelism to the horizon and each other, which it seems difficult lo separate from the supposition ol' their 
having been deposited in water. 
