190 IGNEOUS ROCKS 



mineral, is called amygdaloidal, a term derived from the Greek 

 word for almond. 



2. The Compact (or Felsitic) texture is characterized by the 

 formation of exceedingly minute crystals, too small to be seen by 

 the unassisted eye, giving the rock a homogeneous, but stony 

 and not glassy appearance. If the crystals are too minute to be 

 identified even by the aid of the microscope, it is said to be cryp- 

 tocrystaltine, and when such identification can be made, it is 

 called microcrystalline. 



3. Porphyritic. — In rocks of this texture are large, isolated 

 crystals, called phenocrysts, embedded in a ground mass, which may 

 be glassy or made up of minute crystals. The phenocrysts may 

 have sharp edges and well-formed faces, or they may have irregu- 

 lar and corroded surfaces. The porphyritic texture indicates two 

 distinct phases of crystallization. The first is the formation of the 

 phenocrysts, which remain suspended in the molten mass, or 

 magma, and are often corroded and partially redissolved by it. 

 These crystals are said to be of intratelluric origin, because formed 

 before the eruption of the lava, and such crystals are showered out 

 of certain active volcanoes at the present time. Stromboli (see 

 p. 36), for example, ejects quantities of large and perfect augite 

 crystals. There is reason to believe, however, that not all pheno- 

 crysts are thus intratelluric, but that the first phase of crystallization 

 sometimes takes place after the ejection of the molten mass. The 

 second phase consists in the formation of the ground mass, which 

 may be glassy, minutely crystalline, or both. 



4. Granitoid. — In this texture the rock is wholly crystalline, 

 without ground mass or interstitial paste. The component grains, 

 which may be fine or very coarse, are of quite uniform size, and 

 as the crystals have interfered with one another in the process of 

 formation, they have rarely acquired their proper crystalline shape. 

 Such grains are said to be allotriomorphic. 



An additional texture which should be mentioned is the J rag- 

 mental. This is represented by the accumulations of the frag- 

 mental products ejected by volcanoes (see p. 51), agglomerates, 

 bombs, lapilli, ashes, etc. Many such materials accumulate in 



