OBSERVATIONS AND EXPERIMENTS ON JOINT. PLANES 79 
Joint planes are present in all sizes from an inch to two or three 
hundred feet in length and height but there is not an even gradation 
from one to the other extreme. The joints are divided into groups 
and each group shows distinct characteristics. 
The strike and dip relation of the master joints of this region 
has been generally observed and readings have been made on the 
directions of the joints, chiefly by C. G. Brown whose data were 
used by Professor Hobbs.* 
INFLUENCE OF JOINTS ON TOPOGRAPHY 
The joint planes of this region have a marked influence on the 
form of gorges, cliffs, and waterfalls. The joints in the cliff along 
Cayuga Lake were made famous by the illustrations of Hall and 
Dana. Fig. 8 is a photograph of some of these joints at locality 46, 
Fig. 7. Their effects on gorges and waterfalls have been illustrated 
and described many times.? The rapidity of erosion seems greater 
where the strongest joints are transverse to the general stream 
direction rather than parallel to it. In the latter case small streams 
often follow a very narrow channel between two parallel joint faces. 
Where the transverse joints are strong and there are one or more 
sets nearly parallel to the stream, there are often broad chambers 
in the gorge with the walls formed by the larger joint faces and the 
stream entering and leaving by narrower openings. These may be 
seen in upper Lick Brook and in other gorges in the hard Portage 
rocks. The similarity between some of the larger features of 
drainage and the directions of the joint planes seems to be due to 
the fact that the streams were once consequent upon the same 
uplift with which the joint planes are associated. 
*W.H. Hobbs, Jour. Geol., XIII, No. 4 (1905), 363-74. 
2R.S. Tarr, Bull. Amer. Geog. Soc., XXXVII (1905), 193-212; Pop. Sci. Mo., 
LXVIII (1906), 394-96; U.S. Geol. Surv., Folio 169, p. 3; field edition, pp. 24-25; 
New Physical Geography; Physical Geography of New York State; W. H. Hobbs, Jour. 
Geol., XIII, No. 4 (1905), 363-74. 
[To be continued] 
