Dr. J. W. Evans — Presidential Address. 557 



interesting details of the conditions under which they were 

 deposited. 



It is of special importance to recognize and examine in detail the 

 occurrence of rhythmic repetitions of a similar succession of 

 sedimentary materials and characters. A single cycle in such a 

 succession may be only a twentieth of an inch in thickness, as in the 

 case of ferruginous banding in the Lower Hangman Grits at Smith's 

 Combe in the Quantocks, or may include 30 or 40 feet of strata, 

 as in the Caithness Flags. Rhythms have been described from 

 the pre-Cambrian of Finland, the Ordovician of North America, 1 the 

 Permian of Stassfurt, 2 the Creta"ceous of Arkansas, 3 and the 

 Quaternary of Scandinavia and Palestine, and many more, no doubt, 

 occur in the stratigraphical succession of different countries. It 

 would probably be found that a similar repetition occurs in fine 

 terrigenous deposits off the coast of tropical countries where there 

 is a well-defined alteration of wet and dry seasons. In some places 

 minor cycles may be superimposed on larger, as in the case of the 

 Skerry Belts described by Bernard Smith 4 in the Upper Keuper of 

 East Nottinghamshire. The general question of the significance of 

 such rhythms of stratification must, however, be reserved for another 

 occasion. 



It is more difficult to arrive at the true interpretation of the 

 phenomena presented by the endogenetic rocks 5 which have come 

 into existence by the action of the forces of earth's interior, for the 

 conditions of temperature and pressure under which they were formed, 

 whether they are igneous rocks in the narrower sense, or mineral 

 veins, or metamorphio in origin, were widely different from those 

 with which we are familiar. Under such circumstances the ultimate 

 physical principles are the same, but the so-called constants have to 

 be determined afresh, and a new chemistry must be worked out. It 

 is necessary, therefore, as far as possible, to reproduce the conditions 

 that prevailed — a task which has been courageously undertaken and 

 to a considerable extent accomplished by the Geophysical Laboratory 

 of the Carnegie Institute at Washington. 



By artificial means temperatures and pressures have been already 

 produced far higher than those that were in all probability concerned 

 in the evolution of any of the rocks that have been revealed to us at 

 the surface by earth-movements and denudation, for it is unlikely 

 that in any case they were formed at a greater depth than five or six 

 miles, corresponding to a uniform (or, as it is sometimes termed, 

 hydrostatic) pressure of 2,000 or 2,400 atmospheres, or at a greater 

 temperature than 1,500° C. Indeed, it is probable that the vast 

 majority of igneous and metamorphic rocks, as well as mineral veins, 

 came into existence at considerably less depths and at more moderate 

 temperatures. It is true that most of the rock-forming minerals 

 crystallize from their own melts at temperatures between 1,100° C. 



1 Barrell, Bull. Geol. Soc. Am., vol. xxviii, pp. 789-90, 1917. 



2 Ochsenius, Zeitsch. filr praktisdie Geologie, vol. xiii, p. 168, 1905. 



3 Gilbert, Journal of Geology, vol. iii, pp. 121-7. 



4 Geol. Mag., 1910, pp. 303-5. 



5 Crook, Min. Mag., vol. xvii, p. 87, 1914. 



