MADEIRA. 553 
the lofty walls to their summit. They were similar in form, consist- 
ing of a compact gray or bluish-gray basaltic rock, and scarcely alter 
at all the adjoining rock, except where it is a tufa. 
The following sketches exhibit a number of dikes intersecting the 
cliffs between Machico and Canical. The cliffs consist mostly of a 
hard brownish-red rock, alternating with narrow red layers, and a few 
broad tufaceous deposits of a brown colour. The dikes are from six 
inches to six feet in width: in one place there were nzne in a distance 
Fig. 3. 
of two hundred feet. The dikes were generally transversely fractured, 
showing a tendency to a columnar structure ; but in the broad dike, re- 
presented in figure 3, there were lines of fracture parallel to the walls. 
139 
