88 REPORT-——1905. 
plain which forms the south-western part of Oahu Island, west of Pearl Harbour. 
This plain is about nine miles in length, and of an average width of about two and 
a half miles, and is practically level. The Waianz mountains rise to the west of 
north, the first high summit being 53 miles distant N., 30° W., 2,450 feet; the 
second, 64 miles N., 22° W., 2,740 feet; the third, 7} miles, N. 20° W., 3,110 feet. 
The Observatory is about one mile from the seashore. 
Geological Structure.—The coral plain is a raised barrier reef of great depth, 
estimated at 2,500 feet at the seashore. On the basis of this estimate the depth at 
the Observatory would be about 1,800 feet. The surface is covered with loose coral 
stones of all sizes, with very little soil, and there are frequent large, irregular holes, 
10 or 15 feet deep (some reaching a depth of even 30 feet or more). Water is found 
at a depth of about 45 feet, or about sea-level.! 
Time-heeping,—Star observations with theodolite. 
8. A. DEEL, Magnetic Observer U.S. C. and G. §. 
Kodaikanal Observatory, Madras, India. 
Lat., 10° 13’ 50’’; long., 5h. 09m. 52s. E.; alt., 7,688 feet. 
Foundation is on rock. 
Topographical Sitwation.—On the top of a hill. At a distance of about two miles 
on the east and south sides the hills slope very steeply down to a height of 800 to 
900 feet above sea-level. Towards the west and north the platcau is much more 
extensive. The highest point lies to the W.S.W., is four miles distant, and the top is 
8,200 feet. The Palani Hills, on which Kodaikénal stands, form a mass fifty-four 
miles long from east to west and fifteen miles broad. The plateau is at an average 
elevation of 7,300 feet above sea-level. 
Geological Structure.—‘ Charnockite, ‘a group of hypersthene-bearing rocks, which 
form the largest single section of the Archzean gneisses in peninsular India.’* The 
rocks have been but little disturbed, and there are well-marked lines of false bedding 
running N.E. and 8.W. The chief precipices face either nearly south or nearly east, 
so the chief lines of jointing may be considered parallel to these directions. 
It is an astronomical observatory, but it also receives a direct signal from the 
Madras Observatory clock at 4 P.M. daily.* 
C, MICHIE SMITH, Director Kodaikdnal and Madras Observatories. 
Royal Alfred Observatory, Mauritius, 
Lat., 20° 5’ 39’ S.; long., 3h. 50m. 12°6s. E. of Greenwich; alt., 178 feet. 
Foundation is on alluvium. 
Topographical Situation.—On a plain three miles from the west coast. From N. 
through E. to 8.E. the ground generally rises to Mount Pitou, the summit of which 
bears about E.S.E., and is 917 feet above mean sea-level. Between §.H. and §.W. 
there is a chain of mountains, the highest peak of which, the Pieterboth, bears 
nearly six miles due south and has an altitude of 2,874 feet. 
Geological Structure.—The island is of volcanic origin. It has been supposed 
that the alluvium has a depth of from 2 to 14 feet, below which is solid basalt; but 
I have recently dug 23 feet, to obtain a solid rock foundation for the seismograph, 
and instead of rock I came to water, which has risen 9 feet in the hole. This will 
account for the large changes of level produced by heavyrains. I havealso recently 
discovered that a lamp at night (to check tremors) introduces a change of level, the 
boom tilting away from the lamp. It seems as if the tremors were caused by radia- 
tion of heat from the pier, and I thought our magnetic basement (whose floor 
is 13 feet below the surface of the ground, and in which the diurnal range of tem- 
perature is usually less than 0:3° F.) would be an ideai place for the seismo- 
graph. ‘There is only one spot where there is sufficient room for the instrument, and 
in that spot, as I have said, we came to water at 10% feet, i.e, 233 feet below the 
surface. 
See Brit. Assoc. Rep., 1903, p. 78. 
Holland, Memoirs of the Geological Survey of India, vol. xxviii. 
Also see Brit. Assoc. Rep., 1899, p, 175. 
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