Notes mtd Cojnments. 



35 



a minute laminar structure from which probably the rate of 

 deposition may be learned approximately, a common rate in 

 the older rocks being from o to i8 inches per hour. But com- 

 plex and difficult experiments are very desirable on this ques- 

 tion. The rocks classed as clays differ very much in structure, 

 and must have been formed under different conditions. 



ANCIENT VOLCANIC ERUPTIONS. 



Applying these conclusions to various rocks, the Author 

 shows that in the green slates of Langdale there is good evidence 

 that the volcanic eruptions sometimes occurred within a few 

 weeks of one another, and at other times at more distant 

 intervals. Now and then there were bottom-currents, probably 

 due to volcanic disturbances, gradually rising to a rate of about 

 I foot per second and gradually subsiding, the entire period 

 being a few minutes, and deposition taking place in different 

 cases at from -^q to 2 inches per minute. There is also good 

 evidence that, when deposited, part of the rock was analogous 

 to fine, loose sand, and part to semi-liquid mud. In the Coal- 

 Measure sandstones deposition at the rate of i inch per minute 

 was common, with intervals of little or no deposit. 



MINUTE CAVITIES IN ROCKS. 



The volume of invisible cavities in rocks varies from 49 

 per cent, in some recent rocks, to nearly 0 in the ancient slates. 

 The packing of grains was discussed mathematically and exp^ri- 

 ■mentally, the latter with round and flattened shot ; and experi- 

 ments with sands of various qualities, rapidly deposited and 

 also when well shaken, show a good agreement with calculation. 

 The methods of determining the volume of minute cavities 

 in rocks are given, followed by a number of examples from 

 recent and older deposits. It is found that in some hmestones 

 the cavities have been reduced by pressure to close on the 

 mathematical minimum, whereas in others, even of Silurian 

 age, the cavities were filled with carbonate of lime, introduced 

 from without, not long after deposition. Some oolites have had 

 their cavities filled in a similar manner ; in others most of the 

 material of the original grain has been removed, and the present 

 sqhdity is due to the filling-up of the original cavities mainly 

 by internal segregation. Among fine-grained rocks the Chalk 

 probably was originally a sort of semi-liquid with fully 70 per 

 cent, of its volume water, and in its present state is about 45 



.1908 February i. 



