THICKNESS. 337 



of stratification become more obscure until they are lost in the limestone at the (R. R.) 

 High Bridge. The strata at Winooski Falls probably pass under the quartz ledge just 

 mentioned, although the strata are covered with alluvium at the supposed line of contact. 



" To the northward of Winooski River the red sandstone strata are either concealed, or 

 are mostly without the red coloring matter which characterizes them to the southward. 

 The quartz strata resembling those above Winooski Falls are finely exposed in numerous 

 places to the east and northeast of Mallett's Bay. In one place a ledge of pure stratified 

 quartz, twenty-five feet thick, is exposed. Northeast of these localities the silicious dolo- 

 mites are extensively developed, but are too silicious to make good quicklime." 



There are three localities of the stratified quartz in Colchester : in the range of the red 

 sandrock, southeast of Mallett's Bay, about a hundred rods from the water ; in the sili- 

 cious limerock on the east side of the road, a mile north of the Methodist Church, in the 

 west part of the town ; and at a hundred rods west of the stage road from Burlington to 

 Milton, on the road to Clay Point. 



We have not evidence before us to decide positively whether this quartz rock, with its 

 associated dolomites, passes from the northwest part of Colchester west of Cobble Hill, to 

 connect with similar rocks at Milton Falls, and thus with the group of rocks which we 

 shall describe under the name of talcose conglomerates. We have not ventured to con- 

 nect them upon the map, but can hardly doubt their connection. But the western range 

 passes along the border of Lake Champlain, from Mallett's Bay to St. Albans Bay, as it 

 has been examined all along the coast, and has been crossed twice in Milton, twice in 

 Georgia, and several times in St. Albans. The rock is generally highly colored, and is 

 mostly calcareous. It is massive, sometimes appearing destitute of stratification. At St. 

 Albans it is almost sandstone, between the bay and the central village, with scarcely any 

 dip. It passes now away from the lake east of Rich's quarry, in Swanton, and east of 

 Swanton Falls, growing more silicious, until in the north part of Highgate, on the Tea- 

 chirt hills, it is mostly red quartz rock. But east of the Teachirt hills it is a dark brown 

 silicious limestone, succeeded in turn by a curious breccia, already described. 



The thickness of the red sandrock series, upon Prof. Adams' section of Snake Mountain, 

 is more than 500 feet ; and this does not include the whole of the rock, because a few feet 

 of the higher portion may have been worn away. Prof. Thompson estimated the thick- 

 ness of this group, at Burlington, to be 300 feet. This would give the thickness for the 

 constituent members about as follows, beginning at the top of the series : 



Silicious limerock, . . 75 feet. 

 Brown quartz, . . 50 feet. 

 Calcareous sandstone, . 60 feet. 

 Red sandstone, . . 75 feet. 

 Winooski marble, . . 40 feet. 



We think that the thickness is somewhat greater in the northern part of the State, but 

 do not feel able to state the exact thickness, as all the calculations based upon the ob- 

 served dips give a much larger thickness than seems probable. For instance, the thick- 

 ness of the whole group of rocks between St. Albans Bay and the eastern limit of the tal- 

 cose conglomerates, as calculated, amounts to several thousand feet ; but in Canada these 

 rocks are four thousand feet thick, and our calculations seem to agree with these results. 



