F. W. Clarke — Constitution of the Zeolites. 187 



These figures indicate, it seems to me, that on the whole the 

 fauna of the East has nearly or quite recovered from its 

 enforced removal from the northern States and Canada at the 

 time of the Glacial Period, and that whatever influence the 

 past existence of a Glacial Period may now exert upon the 

 distribution of animal life in North America should be sought 

 only in minor features, such as the remnants of boreal faunas 

 lingering in favorable spots amid temperate surroundings, and 

 the similar features induced by the latitudinal trend of our 

 great mountain chains. 



Art. XXYI. — The Constitution of the Zeolites / by 

 F. W. Clarke.' 



The obvious relationship of many zeolites to the feldspars 

 has long been recognized as a probable key to their constitu- 

 tion ; and yet, so far as I am aware, no systematic theory of 

 the connection between the two groups has been put forward. 

 Partial work, indeed, has been done ; as by Streng in his study 

 of chabazite, and by Presenilis in his interpretation of phillips- 

 ite and harmotome ; but a general explanation of the zeolites 

 all together is still wanting. Much evidence, however, has 

 accumulated with reference to the problem ; such as that 

 derived from the practical dehydration of the minerals, their 

 modes of decomposition when fused, their synthetic formation, 

 their changes under the influence of reagents, and so on ; and 

 the purpose of any exhaustive discussion of their nature 

 should be to bring all these lines of testimony, all these varied 

 data, into convergence. No system of constitutional formulge 

 can be valid unless it fulfils these conditions. It is possible, 

 of course, to seek for too great a generality, and so to force an 

 apparent harmony where real relations do not exist. To avoid 

 this danger, it seems necessary to admit at once that all zeolites 

 are not of feldspathic origin ; some few are compounds of 

 quite a different type ; but most of the members of the group 

 are easily represented by one general set of expressions, which 

 apply to the feldspars as well. 



The theory of the zeolites which is developed in the present 

 paper, is a direct outgrowth and sequence of the views which 

 I have published of late years concerning the micas, the 

 chlorites, and various other minerals, and it is based upon the 

 same principles. The complex or apparently complex alumi- 

 nous silicates are regarded as generally being substitution 

 derivatives of simple normal salts, such as Al 4 (Si0 4 ) 3 , Al 4 (Si 3 8 ) 3 , 

 etc. ; a conception which brings to light many important rela- 



