104 | GEOGNOSY. [Boox II. 
recognizable crystals are scattered. Examined with the microscope, 
this ground-mass is found to present considerable diversity. It may 
be (1) wholly a glass, as In some basalts, trachytes, and other volcanic 
products; (2) partly devitrified through separation of peculiar little 
eranules and needles which appear in a vitreous base ; (3) still further 
devitrified, until it becomes an aggregation of such little granules, 
needles, and hairs between which little or no glass base appears 
(microcrystallitic) ; or (4) “ microfelsitic,” closely related to the two 
previous groups, and consisting of a nearly structureless mass, marked 
usually with indefinite or half effaced granules and filaments, but 
behaving like a singly refracting amorphous body. 
C. Grassy.—Composed of a voleanic glass such as has already been 
described. It seldom happens, however, that rocks which seem to the 
eye to be tolerably homogeneous glass do not contain abundant 
microliths and minute cayerall Hence truly vitreous rocks tend to 
graduate into the second or semi-crystalline type, This gradation 

Fic. 12.—Fiuxion Srrvcrurg mn Ossrp1an. (20 Diameters, See p. 141.) 
and the abundant evidence of traces of a devitrified base or magma 
between the erystals of a vast number of eruptive rocks, lead to the 
belief that the glassy type was the original condition of most if not 
all of these rocks. rupted as molten masses, their mobility would 
depend upon the fluidity of the glass. Yet even while still deep 
within the earth’s crust, some of their constituent minerals (felspars, 
leucite, magnetite, &c.) were often already crystallized, and suffered 
fracture and corrosion by subsequent action of the enclosing magma. — 
his is well shown by what is termed the fluaion-structure. Crystals 
and crystallites are ranged in current-like lines, with their long axes 
in the direction of these lines. Where a large older crystal occurs, 
the train of minuter individuals is found to sweep round it and to 
reunite on the further side, or to be diverted in an eddy-like course 
(Fig. 12), So thoroughly is this arrangement characteristic of the 
motion of a somewhat viscid liquid, that there cannot be any doubt 
that such was the condition of these masses before their consolida- 
tion, ‘The fluxion structure may be detected in many eruptive rocks, 

