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116 Transactions of the Academy of Scieace of St. Lowts 
tical faces and sharp angles due to intersecting joint planes. This 
is well displayed at Johnson Shut-in in the Edgehill quadrangle 
(11). As the felsites vary considerably in texture, the forms 
taken on weathering also vary, gradations being seen between 
the angular shape of the typical felsite and the rounded form 
of granite. 
Basic dikes weather into small, rounded boulders with a 
rough surface which is easily recognized, In many cases the 
dikes are represented only by boulders in the soil. Dake (1!) 
assumed that the presence of basic boulders where there were 
no outcrops indicated that the diabase weathered more rapidly 
than the felsite. This might however, be taken to mean that 
the diabase was so resistant that boulders remained after all the 
felsite had disintegrated. 
An example of a dike that has resisted erosion to a greater 
extent than the felsite is the one in the SW. % sec. 3, T. 33 N., 
R. 4E. which is shown on the Iron Mountain quadrangle (53). 
It is surrounded by sedimentary rocks because it projected 
above the surface on which they were deposited. At Silver 
Mine, a basic dike forms a trench in the granite, indicating that 
it was less resistant to erosion than the granite. Most dike 
rocks are very hard and dense, although thin sections show the 
ferromagnesian minerals to be highly altered in most cases. It 
is possible that the dikes are more resistant to mechanical weath- 
ering and less resistant to chemical weathering than the felsite 
so that their elevation depends on which process has been domi- 
nant. The fact that jointing causes the dikes to break up into 
small blocks may also tend to increase the weathering of the 
dike rock 
Narrows occur where streams flowing on the sedimentary 
rocks cut down until they reach igneous knobs, and are known 
locally as shut-ins. These shut-ins are due to the fact that the 
igneous rocks are more resistant to erosion than the sedimentary 
rocks. Dake (11) describes a number of such shut-ins within 
the Edgehill quadrangle and others occur throughout the igneous 
area, 
