THE NATURAL HISTORY OF IGNEOUS ROCKS. 205 



the deeper the source of a lava of given composition, 

 the more fluid will it be when erupted. The same con- 

 sideration will of course apply to any magma which rises 

 through a considerable thickness of solid rocks, even though 

 it may fail to make its way to the surface. 



Information as to the viscosity of modern lavas, derived 

 from observation of their rate of flow, would be of interest. 

 From the record of a basaltic eruption from Kilanea 

 Becker (12) has deduced a viscosity fifty or sixty times as 

 great as that of water. The data available for such calcu- 

 lations seem to be few and wanting in precision, and are 

 confined, of course, to subaerial eruptions. As regards the 

 diminution in viscosity due to relief of pressure, the experi- 

 ments of Barus go to minimise its importance. He finds 

 that the diminution of pressure must be at least 200 atmos- 

 pheres (or say 2400 feet of rock) to reduce viscosity as 

 much as one degree rise of temperature would do. 



A few words should be said here of the roughly 

 cylindrical mases, sometimes of solid igneous rock, some- 

 times of fragmental accumulations of material, either 

 volcanic or non-volcanic, which mark the actual vents 

 through which volcanic discharges have reached the 

 surface. 1 Of these so-called necks many examples are 

 preserved in the districts of former volcanic activity in 

 this country, and illustrations of numerous cases are given 

 in the two handsome volumes recently brought out by Sir 

 A. Geikie (13). The supposition that the conduits or pipes 

 represented by these bodies have been formed by violent 

 explosive agency is one obviously prompted by the observed 

 facts, but it is of interest to find confirmation of it from a 

 new line of research, as developed in certain memoirs by 

 Daubree (14). The typical instance considered by him is 

 that of the well-known necks in the diamond-fields of South 

 Africa. Seventeen of these are known situated on a straight 

 line some 125 miles long. Each of these has a generally 



1 In a recent paper Captain A. H. McMahon notes on the northern 

 border of Baluchistan a "huge natural pillar " of trachytic ash having a 

 diameter of 100 yards at the base and rising over 800 feet {Quart. Journ. 

 Geo/. Soc, vol. liii., p. 293, 1897). 



