286 



GEOLOGICAL SURVEY 



worthy set of limestones, once coral reefs, occurs at the very 

 base of the group. 



The Trichinopoly or middle group was, in the absence of any 

 important or well-known locality within its area, called by Mr. 

 H. F. Blanford after the district, beyond the limits of which 

 it is not at present known to occur. In the southern part of 

 the area the formations are principally irregularly bedded sands 

 and clays with a small number of limestone beds, and conglo- 

 merates, these latter being characterized by the presence in large 

 quantities of peculiar granite pebbles, not known in the Utatur 

 conglomerates, which consist of gneiss or coral limestone 

 pebbles. In the northern parts, near Alundanapuram and 

 Grarudamangalam, very remarkable beds of shelly limestone 

 become intercalated in the lower part of the series, while fur- 

 ther north still the series is made up of sands, sandy clays and 

 shales in well-stratified beds, with which occur beds of shell 

 limestone, calcareous grit and conglomerate. The shell lime- 

 stone beds at Grarudamangalam are quarried and yield the 

 well-known Trichinopoly marble, an ornamental stone, the 

 beauty of which would be far greater if it were polished after 

 the European method. Uncut specimens often show great 

 wealth of beautifully preserved marine shells of many species, 

 which appear to have been originally accumulated by the 

 local action of currents. 



The Arialur group, the third or uppermost and most 

 easterly sub-division of the South Indian cretaceous rocks, 

 occupies the largest part of the Trichinopoly area ; but, owing 

 to the great development of cotton soil over the surface, is less 

 well seen than the two underlying groups. It is called after 

 the taluk town which stands within its area. Petrologically 

 the group consists chiefly of white unfossiliferous sands and 

 grey argillaceous sands, enclosing minute fossils, in the form 

 of casts, calcareous grit beds and nodular calcareous shales 

 occurring near the base of the group and again in the upper 

 part. These richly fossiliferous beds are separated by a 



