BUILDING AND ORNAMENTAL STONES. 427 



F.— THE POBPHYEIES, PORPHYKITIC FELSITE. 



(1) COMPOSITION AND ORIGIN. 



Popularly any fine-grained, compact rock, carrying larger crystals 

 scattered throughout its mass is called a porphyry, whatever may be 

 its composition. In the present work the term has been restricted to 

 those acid eruptive rocks of pre-Tertiary origin, consisting of a very com- 

 pact felsitic base formed of an intimate mixture of quartz and feldspar 

 and in which one or both of these minerals are porphyritically developed. 

 The groundmass is usually too fine to allow a determination of its compo- 

 sition by the unaided eye, and under the microscope is found to possess 

 that peculiar felt-like structure called by lithologists microfelsitic. The 

 porphyritic crystals are usually of a different color from the groundmass 

 in which they are imbedded, and hence produce the striking effect which 

 has made these rocks so famous in all ages and caused them to be used 

 in the finest ornamentations in spite of their hardness. 



(2) VARIETIES OF PORPHYRY. ' 



Accordingly as the porphyries vary in mineral composition they are 

 divided into two principal varieties : (1) Quartz porphyry, which con- 

 sists of the fine-grained groundmass in which quartz alone or quartz and 

 orthoclase are porphyritically developed, and (2) quartz-free or ortho- 

 clase porphyry, in which orthoclase alone prevails, no quartz appear- 

 ing either porphyritically or in the groundmass. This last variety, it 

 will be seen, bears the same relation to the quartz porphyries as does 

 syenite to the granites. Through an entire disappearance of the por- 

 phyritic crystals, the rock passes into felsite. The porphyries bear the 

 same accessory minerals (hornblende, mica, etc.), as do the granites, but 

 these are usually in such small particles as to be invisible to the naked 

 eye. 



Porphyries, like granites, are of a variety of colors ; red, purple, gray, 

 green, brown, and black of a variety of shades are not uncommon, and 

 when, as is so often the case, the porphyritic minerals contrast in color 

 in a marked degree with the groundmass, the effect on a polished sur- 

 face is very beautiful. 



(3) USE OF PORPHYRY. 



The porphyries are as a rule intensely hard and tough and completely 

 without rift in any direction. As a consequence they are scarcely at all 

 used in this country, although among the most beautiful and indestructi- 

 ble of our rocks. The celebrated porphyries of Elfdalen, Sweden, are 

 wrought into a variety of objects of art, and with exceedingly beautiful 

 effects. Visitors at the Centennial Exposition in Philadelphia will re- 

 call the beautiful large column and inlaid table of this stone that was 

 there displayed. 



