IN THE SYENITE OF BRADGATE PARK. 103 



Thus, for instance, a mineral (e. (j. a felspar) may he formed in a 

 magma, but afterwards, even though the latter continue to lose 

 heat, may be dissolved by increase either of pressure or of the 

 amount of water present *. At one time, for example, quartz may 

 separate before felspar from a magma, at another, as is more usual, 

 the reverse may occur. Htill, as is well known, in most cases a 

 fairly definite order of crystallization exists, and in a holocrystalline 

 mass those which first form are obviously the most idiomorphic. 



Fig. 2. — Diagram illustrating relative amounts of quartz and felspar 

 and of raatrix in " syenite " frou] Bradgate Park ( x ahout 20). 



A rock may be either a glass or not a glass. If the latter, it may 

 be (according to the usual division) cryptocrystalline, mierocrystalline, 

 or crystalline. The second and third practically pass into one 

 another without any real break, the difference being mainly one of 

 size, but perhaps sometimes of completeness in the segregation of 

 the mineral constituents. Very possibly the first also passes into 

 the second ; here, however, differences in completeness of segregation 

 are apparently more significant than those in regard to size. More- 

 over, a ])orphyritic structure maj^ or may not exist in every igneous 

 rock. It maj" be microporphyritic (if a glass) or macroporphyritic. 

 Actual size hardly can be said to matter, so long as one or more 

 minerals are markedly larger than those associated with them. 

 Hence, if, after any one mineral had formed in a magma, the tem- 



frora quite the same point of view, but I may mention that I am conscious of help, 

 direct or indirect, from papers by, and conversation with, Prof. Judd. Mr. 

 Iddings' most valuable memoir ' On Obsidian Cliff, Yellowstone Park,' and that 

 by Dr. Hatch 'On the Spheroid-bearing Granites of MuUnghderg,' Quart. Journ. 

 Geol. Soc. vol. xliv. (1888) p. .^48, contain many useful remarks and references. 

 * See the suggestive remarks by Prof. Judd in the Krakatoa Eeport. p. 4-, and 

 Geol. Mag. dec. iii. vol. v. p. 1, and the excellent summary of Lagorio's results 

 by jMr. Teall, 'British Petrography,' p. 397. 



