166 FOSSILS. 



The Saxicava rugosa (Fig. 74) is stated by our authorities to be less common than the Sanguinolaria 

 fusca, and more frequently seen then Mya arenaria. The shell being thick and strong, is often found in a 

 very good state of preservation. These shells are figured, not because they are new, but that those who 

 study Vermont Geology from our Keport may be assisted. 



Prof. Thompson says that "Mylilus edulis has been found only at a few localities ; but in some cases the 

 individuals are quite numerous. They are seldom well preserved." 



We quote further from Prof. Adams : "The marine brown clay contains more or less of carbonate of 

 lime, and is well adapted for most agricultural purposes, although rather difficult of tillage. Concretions 

 are common, but are usually small and irregular, either resembling scptaria, or of the cylindrical typo 

 of form. The marine sands of this period occur in the vicinity of Lake Champlain, and are most abund- 

 ant in Chittenden county. They extend eastward up the valleys until they reach a level above the 

 submergence of the epoch, where they are probably of fluviatile origin. They seem to be, in the northern 

 part of the valley of Lake Champlain, the equivalents of the brown clay in the southern parts, a current 

 having perhaps drifted the finer materials to the south. Fossil shells, of the species Sanguinolaria fusca 

 and Saxicava rugosa, are common, but more frequently in the lower beds near their junction with the blue 

 clay, where they are exposed by the encroachment of Lake Champlain. A few cases are known, however, 

 where, in strata of very coarse sand, the first named species is found abundantly at an elevation of 150 

 to 200 feet or more above the waters of the lake. Both species are frequently found with the valves 

 together, and are thus proved to have lived and died in the places where they are now found." 



- r The following is Prof. Thompson's account of the sponge found fossil in 



the brown clay. "While digging a well in Alburgh, about four years ago, 

 T at the depth of eleven feet, the workmen came upon a horizontal stratum 

 of what appeared to be mats of hair. It was in quite compact clay, was 

 about two inches thick, and extended over nearly the whole bottom of the 

 excavation. It excited much curiosity, but very little of it was saved. 

 Having obtained a small quantity of it, I sent it in a letter to my friend, 

 Prof. J. D. Dana, who pronounced it, both upon his own authority and 

 that of Prof. Bailey, of West Point, to be fossil marine sponge. The 

 specimen is in Prof. Thompson's collection in the State House, 

 section of Champlain clays, Mr. Hall has given a section of a part of the Champlain clays, which may 



be seen in Fig. 75. For the first two feet there were found yellow sand and soil. 



For the third and fourth feet, there was found brown sand, containing marine shells. The next two and 

 a half feet was blue sand, filled with marine shells. The remainder to an unknown depth was blue clay, 

 which was very pure. 



CHAMPLAIN CLAYS IN CHITTENDEN COUNTY. 



Prof. Thompson prepared a manuscript report upon the Geology of Chittenden County, from which we 

 extract the following account of the Champlain clays. "The pleistocene clays in the county generally lie 

 beneath the sand, and are for the most part very finely stratified. The purer portions of the clay are low- 

 est, and are generally blue. In these portions the fossils are rare. I have noticed them (Nucula Portlan- 

 dica) only at two localities, one just below Winooski Falls, and the other on the bank of Laplop Eiver 

 half a mile east of Shelburne village. At the latter place the clay is remarkably plastic and adhesive, but 

 contains so much carbonate of lime and magnesia, as to eifervesce violently with acids. A remarkable 

 eruption or explosion took place at this locality a few years since. By some force of water or gas beneath 

 the bed of the clay, a portion of it appeared to have been elevated and distended, till it burst through 

 with a considerable report, throwing large quantities of mud into the air, and bespattering the trees to a 

 considerable distance around. And so adhesive was this mud, that it is said to have been seen upon the 

 trees for a whole year. 



21-2/j. 



Brown 

 sand. 



