CHAP. 3.] KARNÜL FORMATION.—JUMMULMUDGOO GROUP. 71 
stone of the Madras Presidency has in some cases been obtained. It is 
generally of a grey color with a blue shade, sometimes nearly black and 
occasionally of pale buff and fawn colors. The whole series of these 
limestones properly consists of several bands differing as to thickness of 
beds, color, and constitution. The band of grey and blue beds is con- 
stant, and distinguishable throughout the whole field; and these may be 
considered as typical rocks of the series. The non-compact and crys- 
talline varieties are rather silicious in their composition, the silica being 
evenly distributed through the rock in most instances; and in others 
distributed apparently in exceedingly thin layers. The blue beds are 
the most crystalline and compact, ringing out when struck with the 
hammer, and breaking with a conchoidal fracture. They weather with 
surfaces resembling certain.corals.* The grey beds are often as compact, 
but they run into flags and earthy beds, while the more compact varieties 
are even more coralloid in their weathering than the blue beds. The 
darker and nearly black varieties are very compact and not so splintery 
* Mr. Foote, who at the time was strongly in favor of the organic structure of this 
variety of the Jwmmulmudgoo limestones, writes as follows with regard to the beds at the 
north-east end of the valley on the northern flank of the Oondootla plateau :— 
“ The surface of the weathered rocks assumes a coralloid structure due to the presence 
of films of silicious matter scattered through the rock and presenting patterns of such a 
character that it is difficult to ascribe them to any but organic causes. These quasi- 
organic structures are of three kinds. The most organic looking of the three presents the 
appearance of circular pores from ~p to } of an inch in diameter, formed in the thin film 
of silicious (?) matter which rises slightly above the general surface, just around the edge 
of the pore. 
* The film is punctured by these pores at irregular distances from 2 toi of an 
inch apart from each other on the average, but not forming any particular pattern among 
themselves. The distribution of the circular pores is in pattern, or rather want of pattern, 
analogous to the exhalant apertures of a sponge. The film which is rarely more than 3, 
of an inch thick, and generally less, is of greyish yellow or pale greyish brown color and 
of generally dull lustre or altogether without lustre, and lies nearly in the same plane as the 
true laminz of deposition. It cannot be traced in the interior of the freshly fractured lime- 
stone; but as the limestone has been neither tested with acids or submitted to micros- 
copical examination, it is impossible to decide as yet whether these films be really organic 
or inorganic structures actually contained in the substance of the rock, or whether 
they be only pseudo-organic modifications of the surface induced by some chemical action 
during the process of weathering from exposure to atmospheric influences; 
(e) 
