42 Evolution and Distribution of Fishes 



Russell's volume gives a most graphic picture of volcanic 

 activity as proceeding in Coenozoic time. But the activity 

 was undoubtedly much greater and more widespread during 

 the palaeozoic epoch. 



As to the nature of the rocks formed by such dusts, Judd 

 says {ig; 89) : "Some volcanic materials, when mixed with 

 water, have the property of rapidly "setting" like concrete. 

 The ancient Romans and modern Italians, well acquainted 

 with this property of certain kinds of volcanic dust and 

 lapilli, have in all ages employed this "puzzolana" as it is 

 called, as mortar for building . . . The cause of the 

 "setting" of "puzzolana" and tufa is that rain water, con- 

 taining a small proportion of carbonic acid acts on the lime 

 in the volcanic fragments, and these become cemented to- 

 gether by the carbonate of lime and the free silica which are 

 thus produced in the mass." Here Judd refers specially to 

 a variety of lime dust produced by pulverization in volcanic 

 cavities of limestone and siliceous rocks and extrusion of 

 this in almost pure lime-silica combination as an Impalp- 

 able dust. Such then would fundamentally differ from rock 

 produced by hardening of granitic dust or of a quite pure 

 lime rock. 



That considerable subsequent chemical activity and re- 

 arrangement may occur in volcanic dust is also noted by 

 Judd (p. 155) as follows: "Scoria and pumice-stones are 

 frequently found to be acted upon by acid vapors to such 

 an extent that the whole of the material is reduced to a 

 white pulverulent mass. In these cases the oxides of iron 

 and the alkalis have united with the sulphuric or hydro- 

 chloric or carbonic acids, the compounds being carried away 

 in solution by the rain water falling on the mass; the 

 materials left are silica, the hydrated silicate of alumina, 

 and hydrated sulphate of lime (gypsum), all of which are 

 of a white color." 



It is well known, and has often been commented on, 

 that such subaerial deposits, with their preceding or ac- 

 companying earth shocks, and emission of noxious gases, 

 rapidly kill organisms over wide areas, and these may then 

 be found as preserved enclosures. Thus Russell (op. cit. 

 p. 2 86) after referring to dust-like deposits of volcanic 



