NATURAL fflSTORY. 



53 



TEAP DIKES. 



rOKPHYRY DIKES. 



IGNEOUS KOCK. 



In the central part of the state, in the 

 talcose slate formation, they are exceed- 

 ingly rare. They are more common in the 

 eastern part of the state; but abound most 

 of all iu the vicinity of Lake Cham plain, 

 and, particularly, in the neighborhood of 

 Bui'liiigton. 



The strike of the various stratified rocks 

 in Vermont is, generally, from a little west 

 of south to a little east of north, while the 

 trap dikes, for the most part, cut through 

 these rocks in a direction nearly east and 

 west. The width of these, dikes varies 

 from three or four inches to five or six faet. 

 The width of the greater part difi'ers but ' 

 little from three feet. They sometimes cut 

 through the rocks (juite obliquely, both to 

 the strike and the horizon, but are more 

 (iommonly neaiiy perpendicular to both. 

 In some cases, the same dike may be traced 

 for several miles, in nearly a straight line, 

 across the outcrop of the strata. In other 

 cases they will terminate suddenly, and 

 commence anew, at some little distance to 

 the right or left, and then proceed onward 

 in the same direction as before. Faults of 

 this kind are of frequent occurrence in the 

 Tiumerous trap dikes, which exist in the 

 Idack shales along the eastern shore of 

 Lake Champlaiu. The accompanying cut 

 represents one of these, as seen iji the bank 

 of the lake at Clay Point, iu Colchester. 



- ..^'-rfw..,- *^.--t!.<;^3:-i The fault is an offset 

 ^.,.. „,>,_,,,.,,.,,,..„, v.. i. ,, ^^^^. about three feet. 



^M The dike is in black 



" :1M slate. The part of the 



•n| bank above it is sand. 



': ^ At Hubbell's Fall, in 



L V, ;)iooski-river, two 



--.: ;.iults may be seen, 



I .-O^^^^p inthe same dike, in 



■ i^^W^ the bottom o f t h e 



Trap Bike. ^'i^'^'"- 



Some of these dikes are very compact 

 and homogenious. Some have a concre- 

 tionary structure, and, by exposure to the 

 weather, separate into spheroidal masses. 

 ( ttliors again exhii)it signs of a columnar 

 structure ; and still others contain nume- 

 rous light colored crystals, giving it an 

 amygdaloidal character. An interesting 

 dike of this character may be seen in a 

 small island in Lake ChampLiin, a little to 

 tlio northward of Colchester point. 



The Porphyry Dikes arc mostly confined 

 to the southwestern part of Chittenden 

 county. Like the traj) dikes, tliey have, 

 in general, an easterly and westerly course, 

 but they are mucli more irregular in their 

 direction, and much less uniform in width. 

 In some places they seem to bilge up in 

 large rounded masses, crowding and crush- 

 ing the slate all around. The color of these 



dikes and intrusive masses, varies from a 

 dark chocolate brown to a light cream color. 

 In some cases, the embedded crystals are 

 very numerous ; in others, they are rare ; 

 and in others still, no crystals are seen, but 

 they appear to consist of a homogeneous 

 mass of feldspathic mineral. 



No part of the state, which has been ex- 

 amined, so much abounds in dikes, both 

 trap and feldspathic, as the northwestern 

 l)art of Shelburne. Pottier 's point is cross- 

 ed by a dozen, at least. At Nash's point, 

 the two kinds of dike are seen together, in 

 circumstances to aiford a clear indication of 

 their relative ages. Their positions may 

 be uudei'stood by the accompanying cut. 



Porphyry Dikes. 



The cut, which represents the western 

 side of the point, exhibits a perpendiculai- 

 face of porphyry, about 11 feet high and 

 some rods in length, resting upon black 

 slate, and covered above by about 2 ft. of 

 blaclc slate and soil. Cutting through the 

 slate, in an easterly direction, beneath the 

 poi'phyry, are two parallel trap dikes, about 

 eight feet apart, and each about one foot 

 wide. Portions of these trap dikes are also 

 found in the slate overlying the porphyry. 

 These facts make it certain that the trap 

 dikes existed in the slate before the por- 

 phyry was thrown up, and that they were 

 broken oif, and parts of them lifted up with 

 the slate by the intrusion of the porphyry. 

 The more recent origin of the porjihyry is 

 also inferred from the fact that it is fre- 

 quently found to have flowed laterally 

 between the strata of the rocks, while the 

 trap is never found to have done so, show- 

 ing that the latter was formed under a 

 much greater incumbent pressure than the 

 former. Trap has been no where found in 

 Vermo]it in the condition of an overlying 

 mass. 



The only purely igneous rock, observed 

 any where in Vermont, on the west side of 

 the mountains, in any other form tlian 

 that of dikes, is in Charlotte. It there 

 forms the hill, south of the Four Corners, 

 and presents a surface of a number of 

 acres. It is, in appearance, intermediate 

 between common trap and porphyry, and 

 most of it is exceedingly hard. The posi- 

 tion of this hill may be seen on the map of 

 Charlotte, given on page 19, it being the 

 hill indicated on the map nearest the local- 

 ity of the fossil whale. 



