THICKNESS. 355 



rock ? should we not look for it, if at all, certainly east of the limestones and sandstones, if they are like 

 those on the west of the quartz rock ? 



"Another thing must be true also, if the quartz rock is the Potsdam, and the limestones and sandstones 

 the calciferous : The limestones on each side of the anticlinal of the quartz rock will be more largely devel- 

 oped where they become united over the anticlinal, north of the sinking down and disappearance of the 

 quartz rock. If the quartz rock is the Potsdam, I certainly shall expect a greater development of lime- 

 stones in Starksboro and Hinesburgh. You have the facts to test this, I have not, as I have not seen the 

 rock directly north of the Hog Back range, up to and across the Wiuooski River. If the quartz rock is the 

 Potsdam, that is the main anticlinal of the State." 



This letter was written in Stockbridge, August 11, 1858. 



The quartz rock, as it appears in Middlebury and Bristol, is described elsewhere. In East 

 Middlebury, on the farm of George Sessions, there is a mass of quartz twenty rods long 

 and fifty feet high. The mountain range of quartz continues unbroken, except at East 

 Middlebury and Bristol. At the latter place the break is very noticeable, forming a 

 marked feature in the scenery, and is seen for many miles around. The Green Mountains 

 appear in the back ground rather higher than the quartz range, which is known in the 

 vicinity as Hog Back Mountain. The village of Bristol is situated at the western base 

 of the northern part of the range, and New Haven River flows through the gap. Whether 

 this gorge has been excavated by the quiet action of the water alone without any fissure, 

 is uncertain ; but probabilities lead us to suppose that the quartz was removed, not by 

 steady erosion so much as by the removal of the numerous jointed blocks of which the 

 formation is full. 



Much of the quartz rock in the valley between Bristol and Starksboro cannot be dis- 

 tinguished from members of the red sandrock series, and talcose schist is associated with 

 it. A few fossils have been found near Rockville in brown quartz. The north part 

 of this formation gradually tapers to a point ; it is mostly semi- vitreous the strata being 

 discerned with great difficulty. It terminates in the north part of Starksboro, the point 

 being enveloped in impure limestone. 



One or two efforts have been made to ascertain the thickness of this formation. A 

 favorable locality for this purpose, because of the horizontality of the layers, is in East 

 Dorset ; and by measurement the thickness was ascertained to be 973 feet. But as the 

 top of the mountain is quartz rock, and as powerful agents of erosion have acted in the 

 vicinity, we know not how much of the quartz has been removed. But it is proved that 

 this formation is at least a thousand feet thick. 



Prof. Emmons estimates its thickness in Clarksburgh and "VVilliamstown, Mass., to be 

 from 1070 to 1170 feet thick. It consists there in an ascending order of the following 

 members : 1, millstone grit, 30 feet ; 2, talcose schist, 70 feet ; 3, sandstone, 100 feet ; 4, 

 talcosc schist, 40 feet ; 5, granular quartz, 400 feet ; 6, quartz and silicious slate, 30 feet ; 

 7, talcose schist, 400500 feet thick. 



Mineral Contents. 



Very few minerals are found in this rock in Vermont. It is really a mineral by itself. The most impor- 

 tant mineral contained in it is hematite, which occasionally occurs in small veins. Several beds of the 

 brown hematite of tertiary age are located upon quartz rock, and may not unlikely be derived in part from 

 these small veins. 



