30 THE MINERAL CONSTITUENTS OF ROCKS. 



under the microscope, the same as changes are determined in objects in 

 botany, zoology, and astronomy. It is not necessary that one should see 

 an acorn grow to an oak, an apple-seed grow to an apple-tree, a lamb to a 

 sheep, a nebula to a star, before he can reason upon the growth of plants, 

 animals, and stars.* It is sufficient to be able to study these in various 

 stages of their growth, in order to make out their history — to examine 

 numerous specimens that exhibit all the various phases of existence, to make 

 out the general life history of the individual. So in lithology the history 

 of the rocks and minerals can be made out as distinctly and certainly as 

 the life history of the individuals in the other subjects before mentioned. 



In applying the principles of thermo-optics to the mineral constituents 

 of rocks, in order to determine at what temperature the rock was formed, 

 it should be remembered that it is only the minerals of the first class to 

 which thej apply. These alone must have been subjected to the heat of 

 the liquid magma and present permanent marks of that action. Of course, 

 nothing can be asserted concerning the temperature to which a liquid has 

 been exposed, from either the thermo-optical or pyrognostic characters of 

 the resulting minerals arising from its cooling under various conditions ; f in 

 other words, the characters of a mineral after it is formed have little or 

 nothing to do with what it was before it was formed, except so f\ir as the 

 relation may have been shown to exist, by experiment and by observation 

 of the same conditions. Investigations iq)on the thermo-optical properties 

 of minerals belonging to the first class would probably lead to interesting 

 results, if care were taken to select such as are typical of the lava. 



It should also be kept in mind that the conditions under which minerals 

 are formed fiom the crystallization of a cooling magma are different from 

 those under which minerals are formed in veins, fissures, and cavities, or by 

 alteration of the rock mass ; and that minerals truly of the second or indige- 

 nous class occupy a very subordinate position in our mineral cabinets. 



* Peircc, Ide.ilitv in the Pliysical Sciences, 18S1, p. 69. 



•f As well predicate what was the temperature of the water glass and hydrochloric acid, from the amount of 

 lieat it takes to fuse the chalcedonic silica which tlicy form under suitable conditions, as to attempt to prove 

 the heat of a liquid magma from the fusion point of some mineral crystallizing out of its cooling solution. 

 The temperature at which a mineral fuses and the temperature at Avhich it formed have no connection with 

 one another, except in the case of crystallization from dry fusion ; if they do, how hot corals must be ! 



