350 THE POPULAR SCIENCE MONTHLY. 



trate the characters presented by lavas which have consolidated at 

 considerable depths below the surface. 



, The microliths have been proved to be the minute elements from 

 which common crystals are built up ; and cases occur in which a group 

 of them may be seen gradually assuming the outward form and internal 

 structure of a crystal, and in other cases crystals may be found which 

 are undergoing a disintegrating action, and are then seen to be made 

 up of minute elements similar to the microliths. 



The same materials which go to form a lava may assume a glassy 

 condition, or that of a rock built up entirely of crystals. Geologists 

 have given distinct names to the glassy and crystalline forms of lavas, 

 which correspond with the five great classes into which lavas have 

 been divided as follows : 



Crystalline forms. Lavas. Glassy forms. 



Granite Rhyolite \ 



Syenite Trachyte f 



Dyorite Andesite C 



Miascite Phonolite J 



Gabbro Basalt Tachylyte. 



The obsidians do not exhibit enough differences to demand a dis- 

 tinction. 



When the large crystals imbedded in granitic rocks and in some 

 lavas are examined with the microscope, they are often found to con- 

 tain numerous minute cavities, each of which resembles a small spirit- 

 level, having a quantity of liquid and a bubble of gas within it. 



In No. 1 of Figure 2 a group of such cavities is represented, one 

 of which is full of liquid, while two others are quite empty ; the 

 remaining cavities all contain a liquid with a moving bubble of gas. 

 In No. 2 two larger cavities are shown, containing a liquid and a bub- 

 ble of gas. In Nos. 3, 4, and 5 the liquid in the cavities contains, 

 besides the bubbles, several minute crystals ; and in No. 6 we have 

 a cavity containing two liquids and a bubble. 



In the largest of such cavities the bubble may be observed to 

 change its place when the position of the cavity is altered, so as 

 always to lie at the upper side, just as in a spirit-level ; while in the 

 smallest cavities the bubbles appear to be endowed with a power of 

 spontaneous movement, and are seen continually oscillating from side 

 to side and from end to end of the hollow, as in Figure 3, where 

 the dark line shows the path pursued by the bubble. These cavi- 

 ties are exceedingly minute, and so numerous that there must be 

 millions of them in some crystals ; indeed, in certain cases, as we 

 increase the magnifying power of the microscope, new and smaller 

 ones continually become visible ; and it has been estimated that in 

 some instances the number of them amounts to from one thousand 

 million to ten thousand million in a cubic inch of space. In many 

 cases the included liquid is water containing saline matters in solution, 



