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DE. H. c. soEBT o:n" THE APPLiCATioisr OF [May 1908, 



which I could calculate the amount of empt)' cavities completely 

 enclosed in the solid rock. Then, by caretul measurement and 

 calculation, I was able to determine approximately the volume of 

 the cavities open to the outside. Combining these volumes with 

 those calculated from the specific gravity I obtained the following 

 results : — 



Table VI. 



From 150 to 160 feet deep 18 per cent. 



643 to 646 „ „ 32 „ „ 



755 to 766 „ „ 20 „ „ 



„ 1080 to 1090 „ „ 29 „ „ 



Mean = about 25 



It will thus be seen that, from the four specimens studied, no law 

 can be deduced connecting the empty spaces with the depth ; but it 

 might be apparent, if all the specimens were studied in the same 

 manner. The mean value is, at all events, of the same order 

 of magnitude as would occur in deposits that were consolidated 

 by internal segregation without being first subjected to any con- 

 siderable pressure, and it affords no certain evidence of material 

 segregation from the outside. The hardness and structure com- 

 pletely prove the great extent of the internal segregation. 



The limestones from Bermuda and Bahama contain analogous 

 cavities ; but their total volume seems much smaller, and some look 

 as if they were spaces originally occupied by bubbles. At all 

 events, both these and the Funafuti specimens diflPer in a marked 

 manner from nearly, if not quite, all the 500 thin sections of 

 British limestone in mv collection. 



Determination of the Amount of Interspaces in 

 Natural Rocks. 



In studying the interspaces in rocks, two courses are open to us. 

 We may determine their volume in the rock as it now exists, 

 which may or may not have any connexion with its original 

 condition; or, by studying a thin microscopical section, we may 

 ascertain their relative volume in the early condition of the rock, 

 before it was materially changed by subsequent deposition or 

 solution of material. The former is much the most important 

 when looking upon the rock as a building-stone, but the latter, 

 when we wish to learn the history of its deposition. 



To enter into full particulars would be almost equivalent to 

 writing a treatise on the microscopical structure of rocks ; and 

 I shall therefore take it for granted that, in the cases here 

 considered, there is no practical difficulty in distinguishing between 

 the materials originally deposited and those subsequently in- 

 troduced by infiiltration. Having then selected a suitable portion 

 of a suitable specimen, a photograph or a careful camera-lucida 

 drawing is made on thin cardboard of uniform thickness, showing 



