216 Beehy Thompson — Use of a Geological Datum. 



conglomerates, and from a conglomeratic grit which, according to 

 Mr. E. B. Tawney, is either Cambrian or Archseau. The latter is 

 emarkable for the uniform minuteness of the fluid inclusions, and 

 or the almost universal activity of the bubbles. For all I can see, 

 I cannot doubt that a careful study of quartzes of different ages 

 would furnish much useful information, bearing both on the age and 

 origin of sands and on the origin of crystalline rocks. 



There are at present so many well-trained petrologists that perhaps 

 it is not too much to hope that some young aspirant to fame will 

 secure a small grant for expenses, collect quartz-veins of every 

 obtainable age, and exhaust all the information that can be squeezed 

 out of them. The most important papers on the subject after 

 Dr. Sorby's classic paper in the Q.J.G.S. of 1858 are the same author's 

 address to the Microscopical Society in 1876, and Mr. Hartley's 

 papers on fluid inclusions to the Royal and Chemical Societies and 

 his Eeport to the British Association in 1876 and 1877. Perhaps 

 the chief thing to bear in mind as a spur to attack these much 

 shirked problems is Dr. Sorby's final word, viz., " there are many 

 things connected with [the subject] about which we know much less 

 than is desirable." 



VI. — The use of a Geological Datum. 

 By Beeby Thompson, F.G.S., F.C.S. 



APROPEE interpretation of some geological phenomena requires 

 that allowance shall be made for differential earth-movements 

 that have taken place since the period of occurrence of the events 

 or conditions under consideration. 



Present differences of level in rocks of the same age may be 

 partly due to actual differences in depth of the sea-floor on which 

 they were deposited, but they may also be the results of subsequent 

 differential earth-movements either of a regional or of a local 

 character, the latter including ' faults.' 



In order to estimate the amount of displacement or differential 

 movement, it is necessary to select a particular rock as a datum. 

 The rock selected should combine, as far as possible, the following- 

 characteristics : — 



1. It should be comparatively thin. 



2. It should have a considerable horizontal extension. 



3. It should combine similarity in physical characters and 

 palseontological contents over a large area, so that uniformity of 

 depth of deposit may be postulated. 



In the district with which I am best acquainted, Northampton- 

 shire, either of three formations would fairly well meet the 

 requirements named above for certain purposes, viz., the EhaBtic 

 Beds, the Marlstone Eock-bed, and the Cornbrash. For a particular 

 object I selected the Marlstone Eock-bed as the most suitable datum, 

 and the results obtained by its use in this manner are, I think, 

 sufficiently interesting to record, since they appear to justify the 

 selection and the method of use. 



