1 INTRODUCTION. 



We explain the origin of all the features by chemical changes 

 similar to those of pseudomorphism in the mineral kingdom 

 that is, by the removal of one, or more, or all of the original 

 constituents of a mineral, and the replacement of the same by 

 some other constituent or constituents. 



Dr. Dawson has always been chary in stating any particular 

 case on which to justify his designation. In the present 

 paper, however, there is actually one ; and it is put in such a 

 way as to invite discussion. Speaking of the " proper wall " in 

 its condition of " aciculse separated by calcareous interpo- 

 lations," Dr. Dawson says we " try to account for this structure 

 by complicated changes, supposed to have occurred in veins of 

 chrysotile subsequently to their deposition." What the com- 

 plication is we are at a loss to understand, as in this particular 

 instance the change is simply due to the chrysotile having been 

 acted upon by " all-pervading " thermal water, containing bicar- 

 bonate of lime a solution which necessarily corroding or 

 decreting the fibres (silicate of magnesia), and penetrating the 

 divisions between them, deposited calcite in the place of the 

 silacid substance wherever it was removed. This is no mere 

 supposition ; for the clearest evidences have been adduced in its 

 favour : and it is no more a complicated phenomenon than the 

 replacement of the calcareous substance of a shell by calcedony, 

 pyrites, miemite, barytes, and other minerals. 



But considering how Dr. Dawson has spoken of our theory of 

 methylosis, it will scarcely be imagined that he has actually 

 committed himself to it in what Dr. Sterry Hunt would say its 

 " extravagant " form. It would appear that he has observed, 

 " in a portion of the fossil," that the calcareous " proper wall " 

 is " entirely replaced by serpentine," but " still retaining 

 traces of the tubuli," and that t( the walls thus replaced could 

 be clearly traced into connexion with the portions of those still 

 existing as calcite." Dr. Dawson evidently must be an 

 ' ' extravagant " pseudomorphist, or none ! He declares that 

 " no replacement of serpentine by calcite is indicated by the 



