568 ERUPTIVE AND METAMORPHIC ROCKS. 



Sc/ior/ Sc/iisf. — A foliated rock composed of Schorl and Quartz. 



Jl/ica SchisL — Often foliated, composed mostly of Mica and Quartz. The mica is 

 generally Muscovite, but sometimes it is a Magnesian Mica. Occasionally felspars 

 occur, in which case the rock becomes gneissic in character, and may even pass into 

 true gneiss. Garnets are common in Mica Schist. Passages have been observed from 

 ordinary slate, etc., into it ; it is therefore spoken of as a Metamorphic rock. A 

 passage of Dolerite into Mica Schist has also been described by Mr. Teall.' 



Sericite Schist. — K Mica Scliist in which the Muscovite is represented by 

 Sericite, an altered condition of that mineral. 



Talc Schist. — Often foliated, composed mainly of Talc (hydrous bisilicate of 

 magnesia), and Quartz. Garnets, Actinolite, Chlorite, Pyrites, etc., are frequently 

 present. As in the case of Mica Schist, passages from this rock into Slate, etc., 

 have been observed. 



Chlorite Schist. — Sometimes foliated, and composed mainly of Chlorite (hydrous 

 silicate of magnesia, alumina, and protoxide of iron), with some quartz, and 

 frequently with some felspathic, micaceous, or talcose material. Magnetite, 

 Pyrites, and Chalybite are also of common occurrence. It sometimes passes 

 into Serpentine, Talcose Schist, Mica Schist, etc. 



Serpentine. — Chemical composition, Hydrous Silicate of Magnesia. This may 

 represent a very highly altered condition of rocks rich in magnesia, such as the 

 Peridotites. Still it is also possible that soluble magnesian salts may at times have 

 been infiltered into rocks whose original constitution was very different, and which 

 may have contained little or no magnesia. Serpentine often contains Chromic 

 Iron, Garnets, and other minerals. It sometimes passes into Chlorite Schist, 

 Talc Schist, etc., etc. In many eruptive rocks minerals, such as Olivine, Augite, 

 Hornblende, etc., have undergone considerable change, and are now only repre- 

 sented by pseudomorphs of Serpentinous material. 



The vesicular structure, so common in some rocks, occurs almost 

 exclusively in those that have been poured out as lavas or ejected 

 as lapilli. It is, however, met with occasionally in the upper 

 portions of dykes, but the structure is not exhibited in rocks 

 which have solidified far beneath the earth's surface (Plutonic 

 Rocks). Vesicular structure is due to the presence of bubbles 

 of gas, steam, air, or fluid, forced into the molten, pasty mass, 

 either prior to or during ejection or emergence from a volcanic 

 vent, or it is due to the development of steam by the passage 

 of heated rock over a damp surface, or it is caused, possibly, even 

 by the actual disengagement of gases generated by the decom- 

 position of substances contained within the rock itself. Minute 

 cavities containing gases and fluids occur, however, in some of the 

 minerals composing Plutonic, as well as in some of those which 

 compose Volcanic rocks. These are, as a rule, purely microscopic 

 in dimensions, and do not impart a vesicular character to the rocks 

 in which those minerals occur. The cavities in Pumice are 

 elongated vesicles, and the vesicular structure is sometimes well 

 developed in Basalts. Vesicles in a rock are often filled by sub- 

 stances such as Calcspar, Greenearth (an earthy variety of Chlorite), 

 Zeolites, etc., which have gained access to their present tenements 

 by infiltration in solution. Rocks spotted with such filled-up vesicles 

 are spoken of as amygdaloidal (because they are often almond- 

 shaped), this adjective being prefixed to the name of the rock {e.g. 

 Amygdaloidal Dolerite). A somewhat similar structure may, how- 

 ever, result at times from the segregation of certain minerals in a 

 rock during its solidification. Still, as a rule, the different origin of 

 these structures is clearly discernible. F. Rutley. 



1 Q. J. xli. 133. 



