DIVISIONAL PLANES. 



313 



Fig. 229 represents a section of the cliff upon the north side of Missisco River at 

 Plighgate Falls, a short distance below the principal fall. It was drawn by Prof. 

 Thompson, Avhose notes are used in the explanation of the figure. 



FIG. 229. 



a Silicious limestone, four feet thick. 

 b Clay slate, one foot thick. 

 c Slaty limestone, two and a half feet thick. 

 d Silicious bed, with concretions, one foot thick. 

 e Black slate with iron pyrites, three feet thick. 

 / Stratum, one and a half feet thick. 

 g Contorted slaty limestone, six feet thick. 

 h Coarse silicious limestone with concretions, twelve feet thick. 

 Alluvium. 



Jc k represents the steep side of the cliff. 



Many of the strata are filled with fine particles of iron pyrites, which appear to be uniformly 

 disseminated through the mass. 



Divisional Planes. 



Jointed planes occur in the beds of limestone most generally, though by no means wanting in the slates. 

 Inadvertently we have no observations to record, but the jointed structure of the limestone at Snake 

 Mountain is as vivid in our memory, as it was to our vision at the time of our visit. They cross the 

 range nearly at right angles, being vertical planes of separation. We have numerous observations upon 

 the position of the cleavage planes of the slates in the following table : 



Section ai Highgat 



Locality. Strike. 



Bridport, east of village, 



Addison, Cobble Hill, N. 10 E., 



Addison, Cobble Hill, N. 10 E., 



Bridport, northeast corner, N. 80 E., 



North Ferrisburgh, 



Shelburne, west of village, 



Shelburne, Pottier's Point, 



Milton, west shore, N. 53 E., 



Milton, northwest corner, N. 53 E., 



St. Albans Point, 



Lonerock Point, 



Welden's Island, N. 40 E., 



Potter's Island, N. 40 E., 



Dip of Cleavage. 



43 E. to 20 E., 

 75 E., 

 60 E., 

 50 S.E., 

 75 E., 

 68 E., 

 42 E., 

 30 S.E., 

 35 S.E., 

 30 S.E., 

 45 E., 

 23E.and70E., 



Dip of the Strata. 



15 E., 

 15-20 E., 



10-15 S.E., 

 12S.E., 

 5 S.E., 

 10 E., 

 6E., 

 6E., 



Authority. 



C. H. H. 



C. H. H. and A. D. H. 



C. H. H. and A. D. H. 



E. H., etc. 



C. H. H. 



C. H. H. 



C. H. H. 



C. H. H. 



C. H. H. 



C. H. H. 



C. H. H. 



C. H. H. 



C. H. H. 



FIG. 230. 



23 E. and 70 E., 



Thus the angles of inclination of the cleavage planes and of the strata differ from each other greatly 

 from 15 to 60. We give, in Fig. 230, a section of Lonerock Point, illus- 

 trating the difference between the cleavage planes and the strata. 



It is owing to the compactness of the redrock that the Point remains, es- 

 pecially in the form of a long bluff. The red rock seems almost destitute of 

 any marks of stratification. Its position, however, is unequivocal. 



Its dip is about 5 or 6 E., while the slate beneath dips about 10 E. 

 The cleavage planes dip about 50 E. In the figure, a a represent the 

 planes of stratification, b b those of cleavage. Above the limestone, c repre- 

 sents fragments of rock and soil. Upon the edges of the strata, d represents 

 fragments of strata and limestone that have been separated from the rock 

 in place; and it was in these fragments that Prof. Thompson discovered fossils, e represents a large block 

 of the magnesian limestone that has been separated from the cliff above. / represents the level of Lake 

 Champlain. The height of the cliff is about fifty feet; the slate varying from ten to thirty feet in thickness. 

 21 



Lonerock Point. 



