Chap. VI.] trichinopoly district — ootatoor group. 81 



Trichinopoly Group. In tlie neighbourhood of Muddam the Ootatoor beds are somewhat fossi- 

 liferous. The flakey gypseous clays ■which form the mass of the beds, are here intercalated with 



Fossiliferous beds of Muddam. ochreous bands of sandstone and shales, full of fossils, chiefly ia 

 the form of casts. The fossils consist of species of Ostrea, Pecten, Inoceramus, Area, Cardium and 

 Corals, with a few Gasteropoda (chiefly Natica), and a small free Spirorbiform annelid, very 

 characteristic of the Ootatoor Group. With these occur also a few Cephalopoda (Ammonites 

 MantelU, and Baciilites teres,) but these latter are not common. The fossiliferous shale beds ex- 

 tend only for a short distance ; they then become merged in the great mass of laminated gyp- 

 seous clays wliich extend from Agaram Northwards to Terany, and which are either unfossUiferous, 

 or are characterised by fossils of a few species, but aboiinding in individuals, and almost peculiar 

 to these beds. The Muddam fossUs resemble, in association and also in the character of their 

 matrix, those of the Odium beds, about 16 miles to the North-east, a resemblance undoubtedly 

 due to similarity of local conditions at the period of their formation. 



In many places the shales, where unfossiliferous, especially the thinner bands, exhibit a 



Zig-zag shales. peculiar zig-zag structure which, like the well known cone-in-cone 



structure, of which indeed it is a modification, appears to be the result of crystallization; 

 only certain beds are affected by it, as may be seen in many examples : — hand specimens, not 

 more than 6 inches square, often exhibit one half the bedded, and one half the zig-zag struc- 

 sture.* The mineral which determines the formation of the latter is carbonate of lime. Shales 

 of this character occur largely in some parts of the Ootatoor Group, and to some extent also in 

 the Trichinopoly Group, especially in the neighbourhood of Alundanapuram. I have noted them 

 on the map as the zig-zag shales. Another lithological peculiarity of similar origin, and occur- 

 ring in association with the above, is a shale of loose porous structure, formed of 

 layers of impure fibrous carbonate of lime, with a general horizontal aiTangement, which 

 are piled loosely on one another so as to leave the mass full of lenticular cavities of 



Concretions. all sizes. A third peculiar lithological form due to the crystal- 



lization of impure carbonate of lime is that of large concretions (Fig. 7), the surfaces of which 

 Fig. 7. Large Concretion in Ootatoor Shales. 







* A specimen of this fibrous shale, analyzed by Mr. Tween, gave 66 '9 per cent, of carbonate 

 of lime. As is the case in the Fontainbleau sandstone, the carbonate of lime appears to have 

 enclosed a large proportion of foreign matter during crystallization. 



