10/2 THE MICROSCOPE IN GEOLOGICAL INVESTIGATION 



porphyries have this corroded appearance ; whilst the porphyritic 

 constituents of the basic rocks (hornblende, olivine, &c.) not in- 

 frequently show the same alteration (vide fig. 804 ; the dotted line 

 marks the original outline). In the case of the hornblende the 

 dissolved portions usually give rise to the formation of small grains 

 of augite and magnetite, which are then found encircling the 

 ' mother-crystal.' Biotite is somewhat similarly affected, and some- 

 times the whole crystal in either mineral may be rendered almost 

 opaque by the separation of minute grains of magnetite. 



The movement of the igneous mass may cause fracture of the 

 crystals owing to strain or to mutual pressure. The pieces of such 

 broken crystals may often be found in one and the same section, 

 sometimes at no great distance from each other. As the magma 

 solidifies, a further development of crystals occurs. The products of 

 this period constitute the ' ground-mass ' of the rock and are usually 

 small in size, the microscope being frequently required for their 

 detection and determination. 



A glass is sometimes produced in the last stage of consolidation, 



FIG. 804. Corroded olivine in basalt 

 of Kilimanjaro, East Africa. 



FIG. 805. Microlites. (After Zirkel.) 



and appears as a base or * setting' to the previously formed minerals. 

 This, however, is usually studded by minute mineral products endea- 

 vouring to crystallise under unfavourable circumstances. Generally 

 speaking, these products are present in tw.o stages of development. 

 The less perfectly developed forms of these are known as crystallites. 

 They occur in a variety of forms hair-like, spherical, &c. and the 

 smaller forms appear to be optically inactive. In some instances, 

 such as those termed ' globulites,' they may be minute segrega- 

 tions of a glassy nature ; in others crystalline aggregates, in which 

 from the extreme minuteness of the constituents and their mutual 

 interference the usual tests fail ; in other cases they may be desig- 

 nated embryonic crystals. 



The bodies belonging to the higher stage of development are called 

 microlites or microliths (fig. 805). They differ from the crystallites 

 in possessing the internal structure of true crystals and in acting on 

 polarised light. The position of the microlites with reference to 

 each other or to the large crystals is frequently an indicator of the 

 movements of the original fluid mass. When streams of microlites 

 are seen lying with their long axes in one direction, this direction is 



