378 CHARACTERS OF CRYSTALLINE LIMESTONE. 



stone is a simple rock, its aspect admits of many variations from 

 unequal admixture with other mineral substances. Of these, the most 

 frequent are mica, talc, and steatite, the latter of which often com- 

 municates a green or mottled colour to the whole rock. Crystals of 

 augite occur in the Island of Tiree, garnets and felspar crystals in the 

 Col de Bonhomme, and tremolite occurs in it in some places. Argil- 

 laceous slate lies upon its laminae. It sometimes assumes a brecciated 

 character, as if composed of limestone fragments, and more rarely it 

 contains fragments of gneiss and mica slate. 



It is used for statuary carving and architectural decoration ; con- 

 tains a great variety of minerals ; and locally is traversed by veins of 

 quartz, felspar, and granite, and by veins of cobalt, galena, and iron. 



Contains no Organic Remains. The limestone associated with 

 gneiss and mica slate is usually destitute of organic remains. Gneiss 

 and mica schist were therefore formerly classed as hypozoic, or beneath 

 the strata which contain evidences of palc&ozoic life. It has been said 

 that if we suppose the crystalline limestones which are devoid of 

 organic remains to have derived their peculiar texture from changes 

 subsequent to their deposition, developed under the influence of water, 

 subterranean pressure, and heat, it is probable that the absence of 

 organic remains may be a consequence of this change. 



Summary. To conclude this discussion, we may collect in a small 

 compass the possible speculations of the origin of the gneiss and 

 mica schist. 



Gneissic foliation is often the original, or the remains of the 

 original lamination, imparted to the mass by the successive accumu- 

 lation of its particles under water, and altered by subsequent 

 action of water, pressure, and heat. These circumstances are suffi- 

 cient in some cases to modify the internal texture, and almost to 

 obliterate the structure, and thus to reconvert the sandstone into 

 gneiss and the gneiss into granite \ in other cases the grains of 

 quartz, felspar, and mica are united together, and new minerals 

 of easier fusion like garnet are generated ; and in other cases the 

 sandstone acquires a superior degree of coherence, and becomes 

 quartzite. This view reconciles the diversity of the gneissic and 

 schistose beds with probable differences of mechanical origin of 

 their materials and degrees of applied heat ; and takes account of 

 the general truth that the foliation of the whole gneissose and 

 micaceous series is parallel to the great axes of earth-movement. 

 The excessive abundance of minute flexures, the occurrence of many 

 cavities, and the frequency of intrusive quartz veins, are observed 

 on and near to the summits of the arches of the laminae. 



