Chap. VI.] trichinopoly district — ootatoor group. 79 



Plant remains in the form of drift wood, sometimes bored by the 



Teredo, abound in certain parts of the group. 

 Plauts. 



The wood is cycadaceous and exogenous. 



Details of the Geological structure. 



With these preliminaiy remarks we may pass on to a more detailed consideration of tlic 

 Geology of the group. I have above observed that South of Kauray and Garoodamangaliun, 

 the Ootatoor beds consist almost exclusively of laminated sandy clays and argillaceous shales, 

 with comparatively few fossils. 



Beds of this character are met with in the broken ground ai'ound the coral-reef limestone of 



Tripatoor, lapping round the highly sloping sides of these ridges, and 

 Tripatoor. , , ,. . m, i t 



where the limestone is wanting, resting on the gneiss. The beds are 



Character of beds. ij. i,- en -i i -n v v. i • 



alternations oi nne sandy and argillaceous sediments enclosing a 



few pebbles and blocks of the limestone in the immediate neighbourhood of the reef ridges, but 

 without any regular conglomerate. Many of the finer sediments, which are quite soft and un- 

 consolidated if crushed by a slight pressui-e between the fingers, split up into flakes of extreme 

 tenuity, a property due to microscopic scales of imbedded mica, aided by veiy slow original depo- 

 sition, which allowed these minute laminae to arrange themselves in an uniformly parallel direc- 

 tion. In common with the Ootatoor beds throughout this part of the country, these deposits are full 

 of kunkur, especially towards their base. This impure nodular carbonate of lime generally fills all 

 the crevices of the beds, and occurs in lenticular patches between them. As in the case of the plant- 

 beds, it is evidently infiltrated subsequently to the deposition of the rocks in which it occurs. 



AU these beds are quite unfossiliferoiis. About 100 yards, or less, to the South of the limestone 

 they are covered up by coarse conglomerates and sands of the Trichinopoly Group, which are 

 full of pebbles, both of the neighbouring gneiss and limestone, and of the yellow argillaceous 

 nodules characteristic of the Ootatoor beds a little further to the Eastward. Crossing the rise 

 covered with smooth cotton soil between the Tripatoor and Sera- 

 or - es o eraganoor. ganoor nullahs, the Ootatoor beds are again met with in the nu- 

 merous little nullahs which drain its Eastern slope. They here begin to assume the ochreous 

 tint and nodular structure characteristic of the finer deposits of the Group, but contain no 

 fossils. "With the exception of some sandy bands, they are finely laminated, and almost 

 unconsolidated ; and dip at an angle of 15° to 20° away from the Northern boimdary. 



North of Seraganoor, the band of Cretaceous rocks increases to about three-fourths of a mile 



in width, of which the Ootatoor beds occupy the Northern half. 

 At Seraganoor. mi n .i t i/.- • t ■. , «„ . , . 



ihe bouthem half is occupied by a thin covering of Trichinopoly 



sands and conglomerates, containing many characteristic fossils, and fuU of fragments of the 

 argillaceous nodules of the lower beds. The unconformity of the two groups is seen more clearly 

 Unconformity of Trichino- ™ ^^^ neighboiu-hood than perhaps elsewhere in the district, 

 poly and Ootatoor Groups. ^nd the distinctive lithologic characters of the two groups are 



unusually well marked. The Ootatoor beds rest apparently on the gneiss of the Northern 

 boundary, and to within a short distance of the Trichinopoly overlap dip irregularly to 

 the Southward. Beyond this the dij) is generally in the opposite direction, but the bedding- 

 presents many irregularities, and it is at least questionable whether the apparent dip 

 is not in a great measure the result of original deposition. Such is, however, ccrtaiulj' not the 



