SALMON — ON ROCKS. 



231 



TThese new constituents, introduced in the course of time, are certainly foreign to 

 the mineral in its original condition, and are on that account to be deducted. 

 Since, however, alterations seldom take place merely by addition, but more fre- 

 quently by loss of constituents, it is likewise requisite that, in the latter case, 

 the quantities lost should be added . . this is seldom possible. . . These examples 

 will siiftice to show the importance of the minute quantities of substances present 

 in minerals, and generally considered as accidental. . . . They then no longer 

 appear as accidental, but indicate the transition of one mineral into others, and lay 

 before us clearly the genetic part of the conversion-process. ... It is but rarely 

 that the chemist is able to produce artificially the changes observed in nature ; 

 and, in order to trace the various stages of these natural processes, there remains 

 no other course for him to pursue tlian to ascertain by analysis the increase of the 

 non-essential and the decrease of the essential constituents ; and, from the nature 

 of the former, to draw conclusions as to the processes which were going on in the 

 mineral when formed. . . . Cordierite is the starting-point of a Avhole series of 

 alterations, finally ending in Mica. ... It will scarcely ever be possible to convert 

 Augite, Olivine, and Hornblende into Sei'pentine in our labonxtories ; but, when 

 we find Sei'pentine in the form of these minerals, this fact is sufticient evidence that 

 such a conversion can take place. ... The Silicates that are most readily decom- 

 posed are generally those containing Hme, protoxide of iron, and manganese. . . . 

 Minerals consisting chiefly of Silicates of Alumina and Magnesia, which are the 

 most stable of minerals, are less liable to decomposition. . . . The Silicates least 

 liable to decomposition are chiefly such as have originated from the alterations of 

 the less stable Silicates ; so that they may be termed the final products of altera- 

 tion. They are not liable to undergo any further alteration by means of the 

 atmospheric agents. They may be either compound or simple Silicates, — Mica, 

 Chlorite, Serpentine, Asbestos ; Steatite, Talc, Clay, or Kaolin. * * * The most 



remarkable product of mineral alteration is unquestionably mica It is scarcely 



inferior to any in stability. . . . These minerals are not the only/waZ products of 

 alteration ; there are, besides. Quartz in its various modifications, oxides, hydrates, 

 and carbonates, incapable of higher oxidation. 



" The cyclical character which is so generally recognisable in the alteration of 

 minerals suggests the question, whether the last-mentioned minerals, which have 

 been spoken of as final j^roduds of alteration, may not really be particular stages 

 of wider cycles of alteration. It is certain that there is a limit to their duration ; 

 those that are most stable among them — the silicates of alumina and magnesia — 

 may under certain conditions become the starting points of other metamorphic 

 processes. If the silicates of magnesia were dissolved and carried away by water, 

 they would take part in the formation of new minerals. There are hkewise means 

 by which the peroxide of iron and quartz may be again brought within the cycle 

 of alteration." 



Descriptive List of the Rock-forming Minerals. 

 The oxygen^quotient is placed after the name of the species in square 

 brackets, thus [ ]. 



The name of each species is followed by its synonyms and varieties. The 

 synonym is separated from the name of the mineral by a comma ; the 

 variety by a semicolm. 



The per-centage of constituents is put in brackets after the chemical symbol 

 of the constituent. Thus K (5) means that the per-centage of soda 

 is 5 per cent. 

 A. Elements and Binary Compounds. 



1, Graphite. The only regular rock-mineral formed of Carbon is Graphite, 

 of which Diamond is the transparent variety. It often contains a 

 variable quantity of iron, and even the purest varieties have traces of 



