138 
Beconls of the Geological Stirvei/ of India, 
[voL. Xiil. 
that there is at this high level barrier a tako-oif of the Palar waters into the 
Cortelliar basin. There is, of course, first the chance that the permeable beds of 
the Cortelliar may not stretch downward to and under Madi’as ; but this is hardly 
to be expected, after what is known of the tailing-up of the water-bearing 
strata of Pondicherry, even though that town be on the north arm of its plain, 
and that there is a well-known tendency of the rivers on the eastern coast to 
trend up to the northwards in their alluvial basins. 
There are, on the other hand, dangers that borings may meet with obstruc- 
Obstacles In tlie way o*" enough at Madras 
of an irregnlar bottom to itself, though it is possible these may not be encountered 
bo expeoteil, ^ clistance west or north of the city : indeed, the 
chances of reaching the Cortelliar beds would be increased the further north 
the trials ■wore made. The line of the Palaveram and Mount ridges of gneiss may 
extend for some distance underneath the alluviums towards the town; in any case 
the mere rising ga-ound itself has a tendency to shelve under the Adyar, and this 
floor would very likely come within the range of shallow borings. Again; it 
appears' that a boring was many years ago made at the then Inland Customs 
House at Madras, three-quarters of a mile from the sea, to the depth of 65 feet, 
which reached the crystalline rocks. The obstacle may indeed have been a boulder 
of those rocks, such being sometimes met with in the alluvium, but the likelihood 
is that it was a sub-alluvial extension of the Mount ridges. 
The section' in this boring was as foliow's ;— 
Elver saml ......... 
Black clay mixed wltb sand and sbales .... 
Blue clay with sand and lime and pieces of Ironstone 
Granite and quartz rubble. 
Clay and gravel mixed with broken granite, quartz, mica, &c. 
Sand and clay 
Light-coloured sand and clay 
Stiff clay 
Pt. Ins. 
3 0 
1 0 
3 6 
5 6 
20 0 
12 6 
0 6 
9 0 
Total 
55 0 
' See Manual of the Geology of India, pt. I, p. 423. 
’ Newbold; Journ. Royal As. Soc., pt. VIII, p. 248. 
