106 



GEOLOGY OF THE SECOND DISTRICT. 

 34. 



Poltdam Sandstoru. 



Calciftroiis Sandrock. 



1. Potsdam sandstone ; 



2. Fuooidal layers ; 



3-10. Calciferous sandrock ; 



4 Drab colored layers, or water lime ; 



5. Cherty ; 



6. Geodiferous ; 



7, Strata fdled with a species of Orthis ; 



8, 3. Encrmal masses , 



8 8 9 10 11 



9. Mass containing the Bellero- 



phon and other univalves ; 



10. Oolilic ; 



11 Chazy limestone. 



Each of the masses noticed in the section will require a few words descriptive of their cha- 

 racters : 



1. The common variety, as above, is a sandy limestone, differing considerably in the amount of sandy matter ; 

 it, however, always efl'ervesces with acids. It is never a clear limestone, and its recent fracture is uneven, with 

 a dull lustre or slightly glimmering. It is unsusceptible of polish, in consequence of an intermixture of earthy matter. 



2. Towards the lowest part of this rock, there are several layers, amounting in all to about ten feet in thickness, 

 which are drab-colored and entirely earthy ; the texture is always finely granular, and weathers rapidly ; a yellow- 

 ish brown crust forms upon it, which frequently, on being struck, falls ofi'. These strata, where the earthy mate- 

 rials are not in too large proportion, form a very good water lime. Masses of sulphate of strontian and calcareous 

 spar are not unfrequent. 



3. Geodiferous masses are quite common ; the geodes contain calcareous spar, crystals of quartz, sulphate of 

 barytes, sulphuret of zinc: sometimes a single crystal of quartz nearly fills the cavity in which it was formed. It 

 is this portion which furnishes the beautiful quartz of Middleville, Little Falls, and Diamond island in Lake 

 George. 



4. Cherty limestone is not unfrequent ; plates of dark colore.! hornstone intersect the rock in various directions. 



The preceding varieties rarely contain fossils ; those which remain to be described, are 

 often filled with organic relics : 



5. Two separate masses of this rock are filled with the genus Orthis, and together, they are not less than forty 

 feet thick. 



6. Encrinal limestone abounds in the upper part ol this rock. There are not less than three distinct masses, which 

 are composed almost wholly of the joints and encrinal rings. It is the only part of the mass which is susceptible 

 of a polish ; these, in consequence of the reddish color ol the encrinal fragments, form a very handsome marble. 



7 A stratum about fifteen feet thick, containing some univalves, one of which appears to be a small species of 

 Bellcrophon ; it is interposed between the two masses which abounds in the Orthis already mentioned. Il is a 

 darker colored mass than either of the preceding, more earthy, and siliceous. 



8. Oolitic, which forms the upper part at Chazy, and lies below the Chazy limestone. The oolite of this place 

 IS in fine particles; but a coarse variety, in masses a fool in diameter, is not unfrequent. No fossils have- been 

 observed in this variety, though immedatcly above and below they are abundant. 



The entire thickness of the calciferous sandrock is between two hundred and fifty, and three 

 himdred feet. It is sometimes absent, or a mass of a doubtful character, and quite irregularly 

 bedded, appears as its substitute. This is the case at the Falls of Montmorcnci, near Que- 

 bec : an irrcgtilar mass, representing rather the birdseye, rests immediately upon the Pots- 

 dam sandstone. 



