NATURAL HTSTORY. 



43 



ISLE LA MOTTE LIMESTONE. 



TRENTON LIMESTONE. 



uplift of Snake mountain, in Addison coun- 

 ty, and in a few other places. j 

 Chazy, or Isle la Motte Limestone. — This 

 is the most important member of the Cham- 

 plain group, and the oldest, which is in 

 much force in Vermont. This rock forms 

 the principal part of the Isle la Motte, the 

 western part of Grand Isle and the eastern 

 shore of the lake, from Charlotte southward. 

 It usually lies in thick, even-bedded strata, 

 dipping, for the most part, slightly towards 

 the east or northeast. It is of a close, 

 compact texture, easily broken into regulai' 

 blocks, and easily sawed, or hammered, and 

 yet sufBciently strong to serve as the very 

 best of building stone. It constitutes, in 

 many places, the shore of the lake, and is 

 in a position highly favorable for quarry- 

 ing, and for transportation by water. 

 Quarries of this limestone have been opened 

 in various places, and it is extensively 

 used for building and other purposes. 

 Some of the best of these quarries are on 

 the Isle la Motte; and among these, Fisk's 

 quarry, on the west side of that island, is 

 probably the most interesting and valuable. 

 This quarry rises directly on the lake shore, 

 and lies but a few rods from the usual line 

 of steamboat navigation through the lake; 

 and the shore is here so bold that the 

 largest vessels on the lake may safely ap- 

 proach it within a few feet, and a very 

 good landing is constructed. The quarry 

 presents a working breast, rising about 35 

 feet above the lake. The strata vary 

 somewhat in their aspect, but they are, in 

 general, of a bluish gray color. The 

 thickness of the sti'ata, varies from eight 

 inches to five or six feet, and each stratum 

 preserves its thickness with great uniform- 

 ity. The general dip of the strata is about 

 4"^ towards the north east. 



Other excellent quarries have been open- 

 ed on this island, of which Hill's quarry, 

 and the Black Marble quarry, on the east 

 side, are the most important. The Isle la 

 Motte limestone, obtained at these quarries, 

 and at others along the shore of the lake, 

 is already extensively used in the construc- 

 tion of buildings and rail road bridges, and 

 considerable quantities are sawed for 

 hearths, or for being polished as marble. 

 The black marble takes a very fine polish, 

 and some of it is exceedingly beautiful. The 

 surfiices of the natural seams and fractures 

 of the strata of this marble, are frequently 

 covei'ed with a black, often iridescent, gla- 

 zing, resembling the surface of anthracite, 

 and it is probably carbonaceous. 



The Isle la Motte limestone abounds in 

 fossils, among which, species of Maclurea, 

 orthoceras and corals are conspicuous, 

 being seen in the worn and weathered 

 surface of the rocks, in great numbers. 





This cut repre- 

 ''^ sents the Maclurea 



■^i^ migna, as it ap- 

 \ pears on the worn 

 -% sui faces of the 

 "a rocks, on the Isle 

 la Motte, and at 

 ^ most places where 

 ffli--'' the rock is found. 

 The Maclureas are 

 Mutluiea magna, spnal shells, re- 

 sembling in form our little fresh water shell 

 called the Planorbis, but they grow to a 

 very large size. When in the surface of 

 the rock, and about half worn away, they 

 frequently present a spiral coil, eight or 

 ten inches in diameter, sometimes having 

 so much resemblance to serpents coiled up, 

 that the early settlers in the valley of Lake 

 Champlain, regarded them as petrified 

 snakes. 



^r^rr-irTT^ 



V 



■'\ 



O) thocei as 



This cut represents 

 the general form of 

 the Orthocerata, as 

 they appear in the 

 weathered surfaces 

 of the Isle la Motte 



hmestones. The number of species found 

 in this formation is very great, and the 

 number and magnitude of the individuals, 

 accumulated at some localities, is remark- 

 ably so. At some places on the Isle la 

 Motte the rocks, for rods in extent, and 

 several feet in thickness, seem to be made 

 up almost wholly of Orthoceras, closely 

 packed together in a limestone cement. 

 .>ome of these are 18 or 20 inches long and 

 G or 8 inches in diameter at the larger end. 

 The interior of these shells is usually filled 

 with calcareous spar, but they are some- 

 times found empty. 



Several species of 

 coral are found in this 

 Jk- limestone. Some of 



w'V' these have a structure 



V; resembling that of hon- 



~-~ ■ cy comb, and hence 



Columnaria alveolata. they have been suppo- 

 sed, by persons ignorant of geology, to be 

 honey-comb petrified. Like the coral reefs, 

 which are now in the process of formation 

 in many parts of the ocean, they are the 

 work of minute insects called zoophytes. 



The Trenton Limestone. — This lies next 

 in the ascending series. It occupies only a 

 small extent of territory in Vermont, but 

 is every where recognized by its character- 

 istic fossils. From near the south end of 

 the lake it extends northward as far as 



