DETERV \ OF ROCKS 6j 



the concretions having been deposited by infiltration in 

 team-holes of the rock, to that when they weather out, 

 riginal cellular aspect of the mass is restored. 

 > Schistose. Rocks of this group arc distinguish- 

 tar arrangement of their component 

 i Is into parallel layers or folia. These layers con- 

 sist sometimes mainly or wholly of one mineral, as in 

 hornblende-schist ; more usually they are composed of 

 two or more minerals, as in mica-schist and gneiss. They 

 may be observed to run into each other and to be as it 

 were welded togeti. they are distinctly crystal- 



line. In many cases they present a wrinkled or crumpled 

 aspect, showing that they have been puckered by strong 



i-ACT. - Without recognisable component 

 crystals or particles, so far as can be made out in the 

 field, but with a close, homogeneous texture. Three 

 leading varieties may be noticed ist, Glassy and 

 resinous ; ad, Horny ; jd, Fine-grained 



.issy and Resinous resembling bottle-glass 

 <>r pit* h. This sub-group includes the natural glasses, 

 as obsidia tone, and basalt-glass. Some hydro- 



carbons, as amber, asphalt and anthracite possess a 

 marked resinous texture ; but their specific gravity and 

 U-h.ivi. >t:r when heated at once distinguish them from the 

 \ 



(jd) Horny having a feebly lustrous, transit 

 character, like flint The chalk-flints and the cherts of 

 older formations are good examples, and the same texture 

 is often exhibited by dcvitrified volcanic rocks such as 

 some rh\uliie<. am! 



