HOCKS. 75 



pact rocks of this class in which hornblende is largely 

 present, and imparts a green color. Basalt is much like 

 greenstone, but is black or grayish-black. It contains 

 s<niall grains of a silicate of magnesia, and iron, called ol- 

 i vine, from its olive-green color. Trachyte is a grayish- 

 white rock composed of feldspar, hornblende, and mica. 

 Clinkstone is a grayish-blue feldspathic rock, which bears 

 this name because it rings like iron when struck with a 

 hammer. The teTm. porphyry is used in reference to the 

 structure of rocks rather than their composition. It is 

 applied to any feldspathic rock that has crystals dissem- 

 inated through it. It is called greenstone porphyry, ba- 

 saltic porphyry, etc., according to the material which 

 makes up the body of the rock, or, as it is commonly ex- 

 pressed, the base of the rock. AVhat the ancients called 

 porphyry is a rock having a base of compact feldspar, 

 with imbedded crystals of feldspar of various sizes, from 

 a very small size up to a length of three fourths of an 

 inch. The name amygdaloid refers also to structure. 

 It comes from two Greek words, amygdale, almond, and 

 eidos, like, and is applied to any rock of the trap family 

 that has in it rounded cavities filled with some mineral 

 different from the base of the rock. The appearance is 

 as if the rock was once in a pasty state, and, when so, 

 these rounded bodies were mixed up with it like almonds 

 in cake. 



153. Tendency of Trappean Rocks to the Columnar 

 Form. In some cases the tendency of rocks of the trap 

 family to take on a columnar arrangement. is shown in 

 the most decided manner. One of the most noted ex- 

 amples is in Fingal's Cave, on the island of Staffa, rep- 

 resented in Fig. 26 (p. 76). Another is the Giant's Cause- 

 way, in Ireland. The columns vary from 20 to 200 feet in 

 height, and are jointed, presenting, therefore, an appear- 

 ance as if they had been built up by putting one pentag- 

 onal or five-sided stone upon another. Where the sea 

 has dashed against them they are more or less worn 



