64 



LITHOLOGICAL GEOLOGY. 



2. The Nature of the Constituent Minerals. — Rocks being mixtures 

 of different minerals, it is of the highest importance to ascertain the 

 species of minerals present, and especially with crystalline rocks. 



The rock may be limestone ; if so, the test with acids given on page 

 bb will decide it. If not limestone, and the rock is coarse-granular, 

 the several kinds of grains must be particularly studied, and their min- 

 eral nature ascertained, the pocket-lens being employed in the exam- 

 inations, and such other helps as may be necessary. If fine-grained, 

 and no transitions to coarser kinds occur in the region (which can af- 

 ford obvious evidence as to the nature of the constituent minerals), it 

 is generally necessary to examine thin, transparent slices by the micro- 

 scope, and further to investigate the composition of the rock by chem- 

 ical analysis. 



The kind of rock being ascertained, the structure and texture of the 

 rock -mass require study. 



3. The Structure of the Hock-mass. — The constituent minerals may 

 be promiscuously mingled as in granite ; or they may be arranged 

 partially in planes, giving it a banded or stratified appearance in a 

 transverse fracture, as in gneiss and mica schist. In the latter case, the 

 rock will break in the direction of the stratification, having what is 

 called a schistose structure. The rock may be thick-bedded, or thin- 

 bedded, even-bedded, or irregularly bedded. On page 79, and beyond, 

 other kinds of structure, that may be looked for, are described. 



4. The Texture of the Roch — Some of the points to be noted as 

 regards texture are (1) whether the rock is coarse-granular or fine- 

 granular, or whether aphanitic (that is, having no grains visible to the 

 naked eye, as in flint) ; (2) whether it is even-grained and homogene- 

 ous, or whether, in the case of crystalline rocks, one of the minerals 

 is distributed in distinct crystals through the mass of the rock; whether, 

 in such a case, the mass of the rock, called the base, is coarse-grained or 

 not. When crystals of feldspar are scattered through the mass, as il- 

 lustrated in the annexed figure (the white spots in which represent the 



Fig. 58. feldspar crystals), the rock is said to be por- 



phyritic, the porphyry of the ancients be- 

 ing a rock of this kind. The whitish spots 

 are often rectangular, or nearly so, and are 

 sometimes twin crystals. The feldspar crys- 

 tals may be orthoclase, or a triclinic feld- 

 spar ; and the base may be orthoclase, or 

 true granite or gneiss, or other kind of rock- 

 material, and either coarse-grained or aphanitic. The term porphyritic 

 is sometimes applied also where pyroxene or hornblende is in distinct 

 crystals in the rock-mass ; but in this case, the rock should be described 

 as porphyritic with pyroxene, or with hornblende. 



