70 CRETACEOUS ROCKS OF 8. INDIA. [PaRT II. § 1. 



the contrary, the reasons I have enumerated fully warrant the inference 

 I have arrived at as to the age of the CuUygoody limestone. 



General considerations on the origin of the Coral-reef limestone. — la the preceding 



paces I have entered at some length into the details of the structure, and of the 



other physical peculiarities of the formation, which I have designated as coral-reef 



_ . . limestone, and I would now explain the grounds upon 



Reasons for assummg ' . . _ 



this origin. which I have so termed it, by instituting a comparison of 



the formation in question with that of the great coral formations of the present day, 

 as described in the able works of Darwin, Dana, Jukes, Nelson, and other authors 

 who have devoted special attention to this subject. It is here unnecessary, and beside 

 Original reefs no longer "ly object, to say aught of the external features of fringing 

 recognizable. and barrier reefs and atols. None of the distinctive super- 



ficial features of barrier reefs with their internal and intersecting ship channels 

 frino-ino- reefs, ledges, &c., are recognizable in the fragmentary ridges which alone 

 remain to us worn down by repeated denudation and half buried in newer depo- 

 sits. The structure of the reef rock in its several varieties 

 b rue ure r . .^ ^^^ which alone concerns us here, and I shall therefore 



confine myself to those portions of the works quoted which treat especially of this point. 

 To summarize in the first place the characters of the Trichinopoly coral formations. 

 Their general external form is that of ridges extending 

 limestones!'^ ^ &^^ a few hundred yards to 3 or 4 mUes in length, the 



ridges in the latter case not being continuous, but brokea 

 up into a succession of smaller ridges by intervals of varying 

 width ; which interruptions to their continuity existed in some cases certainly, if not 

 in all, previously to the deposition of the Ootatoor beds around. The rock forming 

 the base of these ridges is always massive in structure, 

 " ' sometimes with a conchoidal and splintery, occasionally a 



granular, fracture, and in the former case not unfreqiiently containing corals in situ, 

 principally thin encrusting species of Astrcea. The last variety is exclusively con- 

 fined to the basal portions of the ridge. More massive corals also occur occasionally, 

 as at Maravattoor, and with these are associated large Nerinoeas in considerable 

 abundance. The limestone of the upper part of the ridges 

 pper par i =, ■ j^ generally coarser in structure, consisting of calcareous 



sand, firmly cemented, and frequently full of recognizable fragments of corals, shells, 

 &c. Small unbroken shells also sometimes occur in both varieties of the limestone, 

 but these are exceptional. The sandy form of the limestone presents in a few cases, 

 notably at CuUygoody, a kind of bedded structure, as 

 Bedded structure. evidenced by the ranges of the thick slab-like masses which 



course along the ridge running parallel with its major axis, and dipping towards what 



