SECTIONAL TRANSACTIONS.—C. 315 
Their opponents, on the other hand, consider that the balance is maintained by 
the greater or less depth to which the lighter material, the sial, extends. 
Recent observations on the variation of the force of gravitation and the deflection 
of a plumb-bob, and calculations based upon them, tend to show a somewhat closer 
agreement with the figures which might be expected from the latter hypothesis than 
those which would correspond to the former. 
Other speakers : Capt. H. Saaw, Mr. E. Lancaster Jongs, Col. H. G. 
Lyons. 
12. Dr. Freprerick Wautker.—The Chemical Composition of British 
Quartz-Dolerites. 
At least three distinct suites of quartz-dolerites have been recognised in Britain : 
1. A group of bosses and sills round Dundee. 
2. A suite of K.-W. dykes and sills found across the country from Durham to 
Aberdeenshire. 
3. A suite of S.E.-N.W. dykes and sills ranging apparently from the outer Hebrides 
to Yorkshire. 
There is little doubt that the ages of the first and third suites are respectively 
Devonian and Tertiary, but the age of the second suite, though usually given as 
Permo-Carboniferous, is very doubtful. Examples from all three suites often show 
the same assemblages of minerals and may bear strong petrographic resemblances. 
Accurate chemical analyses, however, serve to bring out unmistakable differences of 
composition. Two analyses of Devonian examples show clearly their andesitic 
composition and relation to the adjacent L. Devonian lava flows ; three closely agree- 
ing analyses of Tertiary examples prove them to be doleritic; seven analyses of 
examples from the second suite while agreeing closely are doleritic, and are more 
femic than any of the analyses of Tertiary forms. 
The low alkali percentage of these seven analyses shows that the rocks of the second 
suite cannot have originated through gravitative settling of early crystallised 
constituents of the Carboniferous basaltic magma, unaided by other processes. 
It is suggested that chemical analyses might throw some light on the age and 
affinities of the quartz-dolerites of the outer Hebrides (Jehu and Craig, Tr. Roy. 
Soc. Edin., vol. liii., Part III., pp. 635, 636) and of the E. Fife necks (Balsillie, Geol. 
Mag., Nov. 1923, p. 540). 
13. Prof. L. D. Stamp.—Seasonal Rhythm in the Tertiary Sediments of 
Burma. 
In Burma the Tertiary deposits were laid down in a long narrow gulf by rivers, 
of which the present Irrawaddy is a descendant, draining from the north. A very large 
proportion of the Tertiary sediments, there, show seasonal banding, and in that case 
there is good reason to believe that each double lamina (i.e. of coarse and fine sediment) 
represents a year’s accumulation. The lamine have been counted in many hundreds 
of sections, and the rate of sedimentation calculated. Further, it is noted that finely 
laminated sediments, deposited at the rate of 1 inch in twenty years, grade insensibly 
into rubbly shales for which a similar rate of deposition may be postulated. Coarsely 
banded sands, deposited at the rate of 1 or 2 inches per year, pass gradually 
into false-bedded sands. Preliminary rough calculations give a duration of two 
and a-half million years for the Peguan period in Burma, which, if present views of 
correlation are correct, represents the whole of the Oligocene and the Miocene up to 
and including the Burdigalian. The Peguan beds are enormously thick—at least 
14,000 or 15,000 feet in measured sections—giving an average rate of deposition of 
1 inch in about fourteen years. 
For the use of workers in the field a very simple notation for banded sediments 
is suggested, comprising an initial figure representing the number of double laminz 
per inch, whilst a letter, c or s, shows whether the deposit is predominantly clayey 
or predominantly sandy. Thus a section recorded on a map as 8/20c/7’12s means 
8 feet of banded sediments, predominantly clayey, with 20 double lamine per inch 
resting on 7 feet of banded sediments, predominantly sandy, with 12 double lamina 
per inch. 
