DESCRIPTIONS OF ROCKS. 417 



"Weisselberg (from Rosenbusch), in which the microlites are 

 distinctly crystalline in form, and some give evidence that 

 they are feldspar crystals, others that they are augite and 

 magnetite, and indicate that the rock is intermediate be- 

 tween a glass and a doleryte. Thus there is a passage to 

 ordinary stone. Trap or doleryte has been used for making 

 bottle-glass ; and attempts have been made to manufacture 

 glass directly from a variety of granite containing little 

 quartz. 



Eruptive rocks, that have come up through fissures, 

 often have glassy particles among the stony in the part 

 near the walls of the fissure when not so through the inte- 

 rior of the mass; and many such rocks, covering large areas, 

 have glassy grains among the stony grains, or a glassy mag- 

 ma, because the cooling generally was not slow enough for 

 complete lapidification ; or they have an undefined base, 

 when examined in thin slices, which the microscope does 

 not resolve into crystalline grains. iSuch portions of a rock 

 are described as unindividualued. An unindividnalized 

 base exists in the basalt of Truckee Valley, the character of 

 a slice from which, highly magnified, is given in fig. 7, from 

 Zirkel ; feldspar crystals, of their usual rectangular forms 

 (part of them sanidin), one of the largish crystals of chryso- 

 lite, and smaller irregularly-shaped augites, are imbedded in 

 a base which consists of a glass-like substance ; and in this 

 material there are extremely small globulite grains which 

 are globules of devitrified glass or incipient crystals. The 

 glassy unindividualized base occupies the spaces among the 

 crystalline portions. 



These differences in crystalline texture are of small im- 

 portance compared with differences in mineral and chemi- 

 cal composition. They are results of accidents, and, at 

 the best, lead only to a distinction of varieties among kinds 

 of rocks. The presence of a little glass, or of disseminated 

 large crystals in a porphyritic way, does not make the rock 

 essentially different in kind. If, however, the glassy na- 

 ture is manifest in the external appearance of the mass, it 

 is convenient to call the rock by a separate name. 



Porphyritic rocks are sometimes named as if porphyry 

 was a distinct kind of rock, or as if the porphyritic section 

 of a kind of rock merited special prominence. But, as re- 

 cognized beyond, "felsyte-porphyry " is porphyritic felsyte ; 

 " dioryte-porphyry" is porphyritic dioryte ; " diabase-por- 



