32 PHYSICAL AND CHEMICAL PROPEETIES OP EOCKS 



crystals, usually of feldspar, the structure is said to be porphy- 

 ritic (Fig. 1, PI. 2). This structure is quite common in granite, 

 but is not particularly noticeable, owing to the slight contrast in 

 color between the larger crystals and the finer ground-mass. It 

 is most noticeable in such effusive eruptives as the quartz por- 

 phyries, in which the ground-mass is exceedingly dense and com- 

 pact and of a black or red color, while the large feldspar 

 crystals are white and stand out in very marked contrasts. 

 This structure is so striking in appearance that rocks possess- 

 ing it in any marked degree are popularly called porphyries, 

 whatever may be their mineral composition. The name is said 

 to have been originally applied to certain kinds of igneous rocks 

 of a reddish or purple color, such as the celebrated red porphyry 

 or ''^roseo antico" of Egypt. The word is now used almost 

 wholly in its adjective sense, since any rock may possess this 

 structure whatever its origin or composition may be. 



Glassy rocks on cooling sometimes have developed in them 

 a series of concentric cracks whereby a broken surface shows 

 numerous rounded or globular bodies with an onion-like shell. 

 This structure, which may be visible only with a microscope, is 

 known as perliUc. It is not uncommon in glassy forms of 

 trachyte and liparite. 



Glassy and felsitic eruptives, particularly of the liparite and 

 quartz porphyry groups, frequently show spherulitic masses of 

 all sizes, from microscopic to several inches or even feet in 

 diameter, usually with a well-defined radiating structure, which 

 are due to incipient crystallization. Such are known as spheru- 

 hteSf and hence rocks in which they occur are described as 

 ^pheruhtic,^ 



Concretionary forms may be developed in rocks either as 

 primary or secondary structures. Many of the forms thus de- 

 veloped are peculiarly deceptive, and it may not be out of place 

 to enter into a discussion of their nature and origin with some 

 detail 



On genetic grounds such may be divided into two groups: 

 (A) Primary concretions, formed contemporaneously with the 

 rocks in which they are found, and (B) secondary concretions, 



^ The structure and origin of these forms has been worked out in detail by 

 Whitman Cross. Bull. Philosophical Society of Washington, Vol. XT, 1891, 

 pp. 411-462. 



