OCCURRENCE OF CRYSTALLINE LIMESTONE. 377 



metamorphic action, it was originally a mechanical deposit. The 

 degree of compactness which it exhibits varies extremely, in some 

 cases approaching the loose granular character of sandstone, 1 in other 

 cases it has the density of the quartz of veins. In this latter case it 

 seems that the mass is composed of fragments so firmly united as to 

 suggest the idea of their having been soldered together by fusion or 

 cementation by infiltration, since their deposition from water. Perhaps, 

 also, in some cases, what has been considered as quartz rock may be 

 really an expanded quartz vein. 1 



Minerals in Quartzite. In South America quartzite is the repo- 

 sitory of many rich ores and metals. Native gold is found in Brazil in 

 a stratified quartz rock, and micaceous iron ore appears to be intimately 

 related to quartz rock. The "flexible sandstone" of the same country 

 is a granular rock with drusy cavities containing topaz and amethyst. 



Crystalline Limestone. 



Its Origin. Crystalline limestone is in general stratified ; it fre- 

 quently alternates with gneiss and mica schist, and sometimes retains 

 argillaceous partings ; it was therefore a w r ater-f ormed deposit. Its state 

 of granular or saccharoid crystallisation is due to changes developed 

 since its deposition, and partly occasioned by the action of heat on 

 contained water : this change is more obvious in the deeper-seated 

 than in the newer calcareous deposits. 



The beds of crystalline limestone, whether distinctly stratified or 

 not, are in general detached and limited, and so entirely enveloped in 

 gneiss and mica slate, as to form but subordinate members of those 

 widespread rocks. This fact is in harmony with the occurrence of 

 thin and imperfect limestone bands among the Devonian slates of 

 West Somerset and North Devon, and appears to indicate that in the 

 earlier periods of British geological time the precipitation of calcareous 

 matter was of a more local and limited nature than those depositions 

 which produced the widespread strata of Carboniferous Limestone, 

 Lias, Oolite, and Chalk. 



However we may seek to explain it, the fact is undoubted, that 

 during the deposition of the rocks which were afterwards converted 

 into gneiss and mica slate, a large quantity of calcareous sediment 

 was deposited, not in one uniformly extended stratum, but at 

 scattered points and in unequal quantity. And this irregularity of 

 deposition continues to be observed in an inferior degree in the 

 limestones of Cambrian and Silurian age, which are often lenticular ; 

 but above this horizon the calcareous strata become more abundant, 

 regular, and extensive. 



Minerals in Crystalline Limestone. Though crystalline lime- 



1 Some of the quartz rocks of Scotland and Anglesea have a conglomeritic 

 character. In Garveloch, Von Dechen noticed rounded masses of granite, quartz, 

 and corneous limestone embedded in a basis of clay slate, passing to quartz or 

 mica schist. Mr. Sharpe cautions us that some of the quartz rocks of MacQulloch 

 are of later date (Phil. Trans. 1851). 



