292 REPORT ON NATIONAL MUSEUM, 1886. 



B.— THE MINERALS OF BUILDING STONES. 



Rocks are mineral aggregates. As a rule the number of mineral 

 species constituting any essential portion of a rock is very small, sel- 

 dom exceeding three or four. In common limestone, for instance, the 

 only essential constituent is the mineral calcite; granite, en the other 

 hand, is almost invariably composed of minerals of at least three inde- 

 pendent species. Upon the character of these minerals and the amount 

 of their cohesion is dependent, to a very considerable extent, the suita- 

 bility or desirability of any stone for architectural purposes. Micro- 

 scopic examination will usually result in increasing the ai>parent num- 

 ber of mineral species, and it not infrequently happens that those 

 present, even in minute quantities, are of great economic importance. 



In the arrangement here adopted rock-forming minerals are divided 

 into four classes: (1) Essential; (2) accessory; (3) original; (4) sec- 

 ondary. 



(1) The essential minerals are those which form the chief ingredients 

 of any rock, and which may be regarded as characteristic of any par- 

 ticular variety; e. #., quartz is an essential constituent of granite; with- 

 out the quartz the rock becomes a syenite. 



(2) The accessory minerals are those which, though usually present, 

 are of such minor importance that their absence does not materially 

 effect the character of the rock; e. g., mica, hornblende, apatite, or 

 magnetite, are nearly always present in granite, yet a rock in which any 

 or all of these are lacking may still be classed as a granite. The ac- 

 cessory mineral which predominates is called the characterizing awes- 

 sory and gives its name to the rock. Thus a biotite granite is one in 

 which the accessory mineral biotite prevails. 



(3) The original constituents of a rock are those which formed upon 

 its first consolidation. All the essential constituents are original, but 

 all the original constituents are not necessarily essential. Thus, in 

 granite, quartz and orthoclase are both original and essential, while 

 beryl and sphene, though original, are not essential. 



(4) Secondary constituents are those which result from subsequent 

 changes in a rock, changes due usually to the chemical action of per- 

 colating water. Such are the calcite, chalcedony, quartz, and zeolite 

 deposits which form in the drusy and amygdaloidal cavities of traps 

 and other rocks. 



In the following list is included all those minerals which ordinarily 

 occur in such of our rocks as are used for building or ornamental purposes. 

 In the first column are given those which compose any appreciable 

 part of the rocks, and any one of which may at times become the prin- 

 cipal ingredient or characterizing accessory. The second column con- 

 tains those which, if present at all, occur only in small quantities: 



