274 PRINCIPLES OF STRATIGRAPHY 



nent particles, while structure is the coarser arrangement. It might 

 be serviceable to retain structure as a general term, and to speak 

 of the finer structure of rocks, /". c, that which can be seen in a 

 hand specimen, as texture, while the coarser structural features, 

 generally, visible only in the field, come under the heading of 

 geotectonics, or earth architecture. 



The texture of Endogenetic rocks is either crystalline or amor- 

 phous, each of these having a number of subordinate phases, pecu- 

 liar to one or other of the classes of endogenetic rocks. A type 

 of texture or aggregation of a higher order approaching structure 

 may also be observed at times in the endogenetic rocks, which give 

 them a resemblance to rocks of clastic origin. The three types of 

 these aggregates, spherites, granulites, and pulverites, will be dis- 

 cussed at the end of the section on endogenetic rocks. A brief de- 

 scription of each class follows : 



I. The Pvrogenic or Igneous Rocks (Pyroliths). These 

 rocks result from the cooling of a molten magma, and are crystal- 

 line or glassy, or of intermediate character. The following groups, 

 based on the composition of the magma, are most generally met 

 with : 



Composition Groups. 



I. Granite group, where the composition is that of granite, which 

 when fully crystallized out would result in the formation 

 of quartz and orthoclase with hornblende and biotite or 

 muscovite generally present. 

 II. Syenitic group, where the composition is that of syenite, re- 

 sulting on crystallization in the formation of orthoclase 

 and hornblende, with other accessory minerals generally 

 present. 



III. Dioritic group, where the composition is that of diorite, i. e., 



resulting in the crystallizing out of plagioclase and horn- 

 blende, frequently with some orthoclase, biotite, and other 

 minerals, and sometimes with sufficient silica to form free 

 quartz. 



IV. Gabbroitic group, wdth the composition of gabbro, which on 



crystallization will yield pyroxene and plagioclase, with or 

 without basic minerals, as olivine, etc. 

 V. Ultra Basic group, the component minerals being ferromag- 

 nesian silicates, without feldspar, and in the more basic 

 with olivine. 



The first of these constitutes the acid group with the percentage 

 of silica (SiOo) 68 or over, the second and third constitute the in- 



