254 
Dr Daubeny on the Geology of Sicily, 
at the rate of a little more than three miles per hour against a 
current of the same velocity as the boat ; and that the speed of 
the same boat would be eight miles per hour, when moving with 
a current of which the velocity is four miles per hour. It should 
be remarked, that these calculations suppose the area of the pad- 
dles, and their velocity, to be adjusted to the maximum propor- 
tions in each case ; were it otherwise, the velocity with the cur- 
rent would be increased, and the velocity against the current 
diminished. 
There still remain several important topics for discussion, 
such as the form of the boat, to move with the least resistance, 
the actual resistance depending on the form and tonnage of the 
boat, and the position of the wheels to produce the best effect. 
Art. VII. — Sketch of the Geology of Sicily. By Charles 
Daubeny, M. D. F. R. S. Professor of Chemistry in the 
University of Oxford. Continued from p. 118. 
This recent breccia is seen to rest upon a formation of quite 
a different nature. The superposition I first observed near the 
road between Mazzara and Castelvetrano, where the former 
rock is seen resting on a calcareous marl, devoid of shells, but 
replete with selenites. As we proceed southwards, the gradual 
rise of this stratum brings more frequently to view the subja- 
cent rock, which at Sciacca is seen at the level of the sea, whilst 
the breccia appears on the heights above, where the town itself 
is situate. The same thing occurs at Girgenti, where the brec- 
cia contains very fine turritellas, trochi, and lunulites ; and in the 
interior of the country, where all the most elevated spots are 
crowned with a similar loose shelly stratum, partly calcareous, 
partly arenaceous, always resting upon blue clay, and always 
full of petrifactions. 
Thus the heights of Castrogiovanni (according to Ferrara, 
480 toises, or 2880 feet above the sea), which overlook the val- 
ley of Enna, so celebrated in the mythology of the ancients, and 
the fabled resort of their gods, are composed of this rock, rest- 
ing upon a white calcareous stratum without shells, alternating 
with beds of marl, and this upon the blue clay which constitutes 
