ERUPTIVE AND METAMORPHIC ROCKS. 567 



The rocks in the group marked B. are called Plutonic, since they 

 have originally solidified at a considerable depth from the surface. 

 Some of them form large tracts of land, others occur only over 

 limited areas, as bosses, and dykes. 



The rocks in the group marked A. are clearly Volcanic, since 

 they occur not merely as bosses and dykes, but also as lava flows. 



As with the rocks in the first table, so also with the two groups 

 in this : they probably graduate into one another, and have no 

 doubt often originated from the same deep-seated sources, although 

 the different depths at which they have solidified have exercised an 

 important influence on their physical and mineralogical characters. 



Schistose Eoeks. — These rocks have a schistose or fissile structure, 

 splitting more or less regularly in planes parallel either to bedding, 

 cleavage, or foliation. In many instances they exhibit a foliated 

 structure, and where this is the case the rock usually splits in 

 directions approximately coincident with the planes of foliation. 

 In cases where the rock has originally been deposited as a sediment, 

 the foliation may coincide either with planes of bedding, or with 

 planes of cleavage; but it should be borne in mind that planes 

 of foliation do not necessarily imply the existence, or the former ex- 

 istence, either of planes of bedding or of cleavage planes, although 

 they very frequently do so. 



The following are some of the most common schistose rocks : — 



Volcanic Ashes.^ — In these the mineral components may sometimes be identified 

 with comparative ease, but they are frequently made up of very fine fragments or 

 flakes, which in the main consist of felspathic material, the species of the felspar 

 being occasionally undeterminable, and probably, in many cases, mixed. Frag- 

 ments of Hornblende, Augite, etc., are often present. These rocks are sometimes 

 well cleaved, and afford good roofing slates, as in the English Lake District. The 

 minute structure and mineral composition of some of them have been described by 

 Dr. H. C. Sorby and others. Such ashes may have been ejected from a terrestrial 

 or a subaqueous crater, and in the former case may have been showered down on a 

 land surface or have been deposited under water as sediment. 



Basalt As/i or Basalt Tiiff.—T\\\% rock has much the same mineral composition 

 as basalt. Indeed we may regard it as basaltic matter ejected in the form of ash 

 and lapilli, instead of having been poured out as a lava flow. In some lava flows 

 it is possible that cleavage may have been superinduced, and so have lent a 

 schistose character to the rock, in which case the fissile structure would serve 

 rather to screen than to elucidate its origin. 



Greenstone Asli. — Much of the rock called Greenstone Ash may probably be 

 safely referred to the above. The term, however, as formerly used, was not 

 erroneous, since Basalt and Diorite were then included in the term Greenstone. 



Aplianite Scliist. — A fine-grained rock of eruptive origin, with a fissile structure. 

 This fissile structure may imply lamination, and favour the supposition that the 

 rock originated as ashy matter, and was deposited under water. On the other 

 hand, it may imply cleavage. The affinities of Aphanite Schist are probably 

 basaltic. It is an ill-defined but a vague and useful term. The same remark may 

 apply equally to the expression Volcanic Ash. No serious exception can, however, 

 be taken to the use of these terms in the field. 



Hornblende Scliist. — The components are for the most part Hornblende and 

 Quartz, often showing a foliated arrangement, but sometimes the two mmerals 

 have crystallized out very irregularly, and occasionally the quartz is very poorly re- 

 presented, or is totally absent, in which case the rock becomes Hornblende Rock. 



^ The terms Agglomerate and Volcanic breccia are sometimes used for these 

 ejectamenta. They are also spoken of as Pyroclastic Rocks. 



