226 STRUCTURE COMMON TO ALL ROCKS. 



cate their origin. Thus he recognizes a syenite and a metasyenite, a 

 diorite and a metacliorite, dolerite and metadolerite, felsite and meta- 

 felsite, etc., and we might add granite and metagranite. 



Many geologists push these views so as to include also even the 

 true lavas. Deeply-buried sediments under gentle heat in the presence 

 of water and pressure undergo incipient change and form metamorphic 

 rocks; under greater heat become pasty and form granite, meta- 

 syenites, metadiorites, metafelsites, etc.; under still greater heat, in- 

 creased probably, as Mallet suggests, by mechanical energy in crushed 

 strata being converted into heat, become completely fused, and are then 

 outpoured upon the surface either by the elastic force of the steam 

 generated, or by the pressure and squeezing produced by the folding of 

 the crust of the earth, so common in mountainous regions. According 

 to this view, every portion of the earth's crust has been worked over and 

 over again, passing through the several conditions of soil, sediment, 

 stratified rock, metamorphic rock, and igneous rock, perhaps many 

 times in the course of the geological history of the earth, and we look 

 in vain for the primitive rock of the earth's crust. 



CHAPTER V. 



STRUCTURE COMMON TO ALL ROCKS. 



We have thus far given a brief description of the three classes of 

 rocks, their structure and mode of occurrence. There are still, how- 

 ever, several important kinds of structure which are common to all 

 these classes of rocks, and require description. These axe joints, fis- 

 sures, and veins. Mountain-chains, as involving all kinds of rocks and 

 all kinds of structure, and as summing up in their discussion all the 

 principles of structural and dynamical geology, must be taken up last. 



Section 1. — Joints and Fissukes. 



Joints. 



All rocks, whether stratified or igneous, are divided, by cracks or 

 division-planes, in three directions, into separable irregularly prismatic 

 blocks of various sizes and shapes. These cracks are called joints. In 

 stratified rocks the planes between the bedding constitute one of these 

 division-planes, while the other two are nearly at right angles to this 

 and to each other, and are true joints. In igneous rocks all the 

 division-planes are of the nature of joints. In sandstone these blocks 

 are large and irregularly prismatic ; in slate, small, confusedly rhom- 



