282 TKENTON LIMESTONE. 



5. Light blue compact schistose limestone. 



6. Ordinary limestones metamorphosed into white, grayish white, and dirty looking ferruginous strata, 

 frequently with a network of veins of calcite or quartz. 



7. Bituminous and fetid layers. 



8. Sandy limestones. 



Perhaps other unimportant varieties might be added to the list, were it desirable. 

 The most common of all are the second and third, interstratified with each other ; and 

 they may be seen at almost any exposure of this rock, between West Haven and Charlotte. 

 Where the rock passes into Utica slate, the first variety abounds, uniformly perhaps, 

 at all events we cannot distinguish the upper part of the Trenton from the Utica slate 

 except by its characteristic fossils. The black limestone is quite compact in structure, 

 and is quite pure carbonate of lime, breaking with a conchoidal fracture. It contains 

 fossils, sometimes quite scantily ; but in this case the fossils are well formed and very 

 beautiful. 



None of these varieties are suitable at all for building stone, except the fourth variety, 

 and that is not very common. There was more of it seen upon the west side of South 

 Hero than elsewhere, where it abounds in very fine fossils. In Canada, this variety is 

 used extensively for building purposes. 



The fifth variety is found in those regions bordering upon a metamorphic rock, as at 

 Highgate Springs, where it greatly resembles some varieties of the sparry limestone of 

 other groups. 



Metamorphism has changed these limestones into white limestones of great purity, at 

 Highgate Springs, where it is burned for lime. Near these beds the limestone is hard, 

 thick-bedded, and compact, much like some varieties of Chazy limestone. Even layers of 

 crystalline mica schist may be seen among them. In the south part of the terrain, in 

 Benson and Orwell, the limestone is utterly unrecognizable, and hard specimens might 

 pass for the metamorphic Eolian limestones. Some of the beds, also, could not be distin- 

 guished from some varieties of the red sandrock series in Highgate, etc. So true is it that 

 metamorphosis totally alters rocks, and we are obliged to name rocks by conjecture if we 

 judge by lithological character alone. Most of the unaltered varieties are more or less 

 fetid, and many are so bituminous as to blacken the hands, or any white substance, rubbed 

 upon it. 



The sandy limestones were seen only at Highgate Springs, and that in limited amount. 

 They correspond in structure with varieties of both the lower groups. The lithological 

 characters of the Trenton limestones, like its fossils, are better marked than those of any 

 other group in the whole Silurian system. 



Divisional Planes. Cleavage planes are common in this limestone. They are not generally perpen- 

 dicular to the strata seams, but cross them transversely at various angles. As this subject has received 

 little attention in the progress of the survey, it is only occasional observations that can be presented. Only 

 two measurements were taken. One at McNeil's Point in Charlotte, gave the strike as N. 20E. and the 

 inclination 70E. Another at Porter's Landing in Grand Isle, gave for the strike N. 70E. and for the dip 

 38S. These measurements represent their average position throughout the whole formation in Vermont. 



Jointed scams are common. They usually cross the strike at right angles, and are perpendicular, at 

 intervals varying from two or three inches to ten feet. 



