CLARK: NEW FOSSILS FROM NEAR BOSTON. 485 
made a certain kind of impression at this level, another a 
few millimeters above, others still further above, and finally, 
perhaps, the original form still higher. Under the escaping-gas 
hypothesis these differences may be explained on the basis first, 
of variations in the amount of escaping gas; and secondly, varia- 
tions in the viscosity of the mud of successive strata. There still 
remains to be explained the peculiar bilateral symmetry of the 
markings. They are seldom, if ever, found with a circular out- 
line, as one would expect if the water were perfectly still. If, 
however, there were a current, one would expect to find these 
markings drawn out in the direction in which the current was 
flowing. The most obvious reason is that the rocks have been 
distorted, compressed in the direction of the smaller diameter of 
the markings. It is well known that the slates of Slate Island 
have been sheared in two directions at two distinct times, so that 
one set of shear phenomena cuts across an earlier one. The pre- 
Cambrian rocks of Newfoundland, in common with most other 
rocks of like age have not escaped this deformative process. 
