THE LAUEENTIAN LIMESTONES. 



271 



Hypersthene is very generally disseminated in these fel- 

 spathic rocks, but always in small quantity. Titanic 

 iron-ore is also found in them, in a great number of 

 places, sometimes in small grains, but often in consider- 

 able masses. 



" With schists and felspars are found strata of quartzite, 

 associated with crystalline limestones, which occupy an 

 important place in this formation. These limestones 

 occur in beds of from a few feet to 300 feet in thickness, 

 and. often present a succession of thin beds intercalated 

 with beds of gneiss or quartzite ; these latter are some- 

 times quartzite conglomerates, and have in certain cases 

 a base of dolomite. Associated with these limestones, 

 are sometimes found beds composed in great part of 

 wollastonite and of pyroxene, species which evidently 

 owe their origin to the metamorphism of siliceous lime- 

 stones. Beds of dolomite and limestone more or less 

 magnesian, are often interstratified with the pure lime- 

 stones of this formation. 



" The limestones of this system are rarely compact, 

 and most frequently are coarsely granulated. They are 

 white or reddish, bluish or greyish, and these colours are 

 often arranged in bands which coincide with the stratifi- 

 cation. The principal mineral species met with in these 

 limestones, are apatite, fluor, serpentine, phlogopite, sea- 

 polite, orthoclase, pyroxene, hornblende, wollastonite, 

 quartz, idocrase, garnet, brown tourmaline, chondrodite, 

 spinel, corundum, zircon, sphene, magnetic and specular 

 iron, and graphite. The chondrodite and graphite are 

 often arranged in bands parallel with the stratification. 

 Beds of a mixture of wollastonite and pyroxene are 

 sometimes met with, which are very rich in zircon, 

 sphene, garnet, and idocrase. The most crystalline varie- 

 ties of these limestones often exhale a very fetid odour 



