16 



pseudo-spherulites. They are especially characteristic of the granophyres 

 (Rosenbusch), of which the rock from the top of Carrock Fell, Cumberland, 

 may be taken as the type in this country. 



CRYSTALS, CRYSTALLINE GRAINS, &c. When a definite chemical com- 

 pound crystallises out of a molten magma it assumes, under certain conditions, 

 a definite geometric form, which is characteristic of the substance under those 

 conditions. It very often happens, however, that the larger crystalline con- 

 stituents of igneous rocks are devoid of this definite external form. This is 

 due to a variety of causes. Where a number of minerals are simultaneously 

 produced in juxta-position mutual interference prevents the development of 

 perfect crystalline forms. This is well seen in the case of plutonic rocks ; such 

 as gabbro, diorite and granite. When the minerals in a rock have been formed 

 at different times, those first produced usually show the most perfect develop- 

 ment of crystalline faces. 



Mechanical actions frequently break up the first-formed crystals in 

 an igneous magma and accordingly we find, in many lavas and tuffs, 

 the fragments of once perfect crystals. Again, the imperfect forms of 

 certain minerals, as for instance the quartz grains of certain rhyolites and 

 quartz-felsites and the hornblende-crystals of certain andesites and porphyrites, 

 is probably due to a corrosive action of the magma on more or less perfect 

 crystals. The process of crystal-building in a mass of molten material is, in 

 most cases, extended over a long period of time during which the chemical 

 and physical conditions are constantly changing. Crystals, like organisms, 

 stand in the closest relation to their environment, and are liable to be 

 modified by every change in the latter. It is not surprising, therefore, to find 

 evidence of such modifications in the internal structures and external forms of 

 the mineral constituents of igneous rocks. 



A typical crystal should be perfectly homogeneous. It rarely happens, 

 however, that this condition of things is actually realised. Inclusions of 

 foreign matter are usually present, and as these often throw important light 

 on the conditions under which the crystal was developed, or on the metamor- 

 phic changes to which it has been subjected, since its formation, they are of 

 great interest to the petrographer. In order that conclusions may be drawn 

 as to the origin of rocks from the phenomena presented by the inclusions in 

 the minerals of which they are composed, it is obviously necessary that the 

 minerals should have been formed at the same time as the rocks, and that the 

 inclusions should have been formed at the same time as the minerals. Now 

 there are original and secondary minerals and there are original and secondary 

 inclusions ; and it is not always possible, in the present state of our knowledge, 

 to be certain as to which of these classes any particular mineral or inclusion 

 belongs. Again, if we agree that an igneous rock attains individuality at the time 

 of final consolidation, then the original minerals belong to two classes those 

 formed at the time of consolidation and those formed anterior to it. If the 

 semi-liquid mass has moved since the development of the former then the 

 inclusions which they contain throw no light on the conditions of final 

 consolidation. Great care is therefore necessary in drawing inferences from 

 the phenomena of inclusions. 



