THE BUILDING OF AN ISLAND. 57 



CHAPTER VII. 



. Minerals. 



The studv of minerals is an extensive subject in itself, nevertheless it is - 

 not difficult to acquire a few simple facts about them which will add interest 

 to our observations when examining the various rocks of our island. 



What is a mineral? That is a (juestion not so easy to answer as mav at 

 first sight be supposed. In every-day language an3'thing found in the solid 

 part of our earth may be called a mineral; but in a scientific sense the word 

 has a more restricted meaning. The following definition and comments are 

 taken from the introduction to Nicol's Elements of Mineralogy: "In the 

 strictest sense, a mineral species is a natural inorganic bodv, possessing a defi- 

 nite chemical composition and assuming a regular determinate form or series 

 of forms. This definition excludes many bodies often regarded as minerals: 

 as, all the artificial salts of the chemist, all the inorganic secretions of plants 

 and animals, all the remains of former living beings now imbedded in rocks. 

 Some substances originallv organic products have indeed, by common consent, 

 found a place in mineral systems, as coal, amber, and mineral resins; but this 

 is a departure from the strictness of the definition, and in most cases had, per- 

 haps, better have been avoided. So also some amor[)hous substances, with 

 no precise form or chemical composition, as some kinds of clay, have been in- 

 troduced into works on mineralogv, but we believe often improperly, and with 

 no beneficial result. Aggregates of simple minerals, or rocks, are likewise ex- 

 cluded from this science, though the various associations of minerals, their 

 modes of" occurrence, and their geological position, are important points in 

 the listing of the different species." 



The same author, in the beginning of his first chapter, in which he treats 

 of the forms of minerals, says: "Mineral substances occur in two distinct 

 modes of aggregation. Some consist of minute particles simplv collected to- 

 gether, with no regularitv of structure or constancy of external form, and are 

 named amorphous . '*" ''' ' '' The other class have their ultimate atoms 

 evidently arranged according to definite law, and are named crystalline when 

 the regularity of structure appears only in the external disposition of the parts, 

 and crystallized, when it also produces a determinate external form or a 

 crystal^ 



In the limestone and marl formation of Santa Cruz and in the few lime- 

 stones of our oldest formations we may find a partial illustration of the above. 

 Frequently in the cracks of the limestones and in the small hollows in fossil 

 corals we may see an abundance of small crystals of carbonate of lime, crystals 

 which are known as calc-spar or calcite . The same substance is abundant in an 

 amorphous state (that is to say, without definite form), in the numerous concre- 

 tions of some of the beds and also in some of the more compact beds in their 

 entirety. The same crystalline limestone is, however, reser\'ed for those spark- 

 ling limestones known as marble, which have been produced in the earth by 



