254 STRUCTURE AND TEXTURE OF LAVAS. 



that the crystals can only be recognised under the microscope. No 

 perfect boundary can be drawn between perfectly crystallised rocks 

 and those in which a minute quantity remains of the original material 

 which has not been differentiated into crystals. 



(2.) Semi-crystalline Rocks. The semi-crystalline rocks are a 

 large class, in which the greater part of the material is an amorphous 

 substance, through which are scattered crystals that may be either 

 microscopical or macroscopicaL The crystals may be few, or so 

 numerous as to form nearly the whole of the rock. These semi- 

 crystalline rocks present several varieties, according to the condition 

 of this uncrystallised material. 



First, it may be purely glassy, consisting of glasses which yield 

 no colours in polarised light. 



Secondly, the glass may be partially devitrified, by the formation 

 of grains and needles. The needles are usually black and hair-like, 

 the grains angular or rounded. They have been termed crystallites and 

 globulites, but neither polarise, and they are both therefore regarded 

 as glass richer in iron. The dark needles or trichites are aggregated 

 into branched or net-like masses. The globulites are common in 

 dolerites and other basic rocks, but rare in rhyolites. 



Third, devitrification may proceed so far that the glass is entirely 

 replaced by such small bodies as those described. 



Fourth, the microfelsitic mass sometimes presents an amorphous 

 substance, free from glass, without transparency, and incapable of 

 being resolved into separate particles. This condition is more charac- 

 teristic of the quartz porphyries and rhyolites than of basic rocks. It 

 will readily be understood that it is often difficult to distinguish the 

 original nature of a rock which has undergone some of these phases of 

 devitrification ; and chiefly on this account, the volcanic rocks of the 

 Primary period have only recently been shown to be essentially the 

 same as those of the Tertiary period, but somewhat decomposed. 



(3.) Uncrystalline Rocks. The uncrystalline type consists of a 

 volcanic rock which was originally amorphous, sometimes glassy, like 

 obsidian or tachylyte, and often in the microfelsitic state. Between 

 these rocks, therefore, and the semi-crystalline rocks there is a com- 

 plete transition. 



Ground Mass and Base. Zirkel uses the term ground mass to 

 indicate the part of a rock between visible crystals which shows no 

 structure to the unaided eye ; but when the rock is examined under 

 the microscope, and a similar homogeneous appearance is seen between 

 crystals, that undifferentiated paste is named the base ; and it is this 

 base which presents the varied conditions of texture which we have 

 enumerated in semi-crystalline rocks. 



Fluxion Structure. Fluidal structure is a term applied to more 

 or less glassy rocks, in which streams of microliths or needle-like 

 crystals undulate and bend in their arrangement about a larger crystal. 

 Such fluxion structure is often seen in the least crystalline basalts, 

 trachytes, and phonolites. These conditions are best observed under 

 the microscope between crossed nicols, and under a low power. The 



