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organic remains, Helix, etc. M. Michel Levy thought the conver- 

 sion of the basalt-magma into fragmental material, was perhaps 

 due to the rapid escape of water from the magma, in regions where 

 the containing walls of the dyke offered little resistance. It is often 

 difficult to draw any line between the solid basalt and the marginal 

 breccia, which has been thrust laterally into the weak structures. 

 Sir A. Geikie remarks: "The material of the peperites has un- 

 doubtedly here and there filled up the volcanic vents, and has 

 been injected in veins and dykes around their margins. But the 

 main mass of the material was ejected from the vents, and falling, 

 as volcanic dust and sand . . . became interleaved with the con- 

 temporaneous sediments" (Textbook of Geology, 4th ed., p. 1255, 

 footnote). It seems reasonable to suppose that, if loose marls below 

 solid limestones, permit a magma to break up into pyroclastic 

 material, such an action will be even more favoured in loosely con- 

 solidated sediments without a compact covering layer. To this 

 extent, the French peperites help us to understand the. features of 

 the Tamworth tuffs. One cannot, however, decide to what extent 

 the pulverisation of the consolidating magma was brought about 

 by the escape of magmatic water, or by the constant forward move- 

 ment of the magma, or by the strains set up, as in a Prince Rupert 

 drop, by the rapid chilling of the melt. The general absence of 

 recognisable points of eruption makes it also impossible to esti- 

 mate the distance to which the intrusive tuffs have been thrust 

 laterally into the sediments. But though clearly intrusive tuffs 

 have been found at many points, it is probable that the fossili- 

 ferous tuffs and breccias deposited in the normal fashion constitute 

 the greater part of the pyroclastic rocks in the Great Serpentine 

 Belt. 



[Printed oflF, 25th November, 1915.1 



