484 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
three-quarters as wide as long, and show no recognizable arrange- 
ment over the surface of the rock. I have been able to compare 
these with specimens of Aspidella from the pre-Cambrian rock 
of St. John's, Newfoundland, and with the exception of the much 
greater size of the specimens from that locality, the only essential 
difference is that ours are much flatter. 
Doubt has been cast upon the organic nature of Aspidella. 
The writer does not hesitate to state that he believes it in- 
organic in origin. A tentative explanation is that these struc- 
tures represent the sites of vents from which gas escaped, but 
not as large bubbles, for these usually make deep crater-like 
depressions, as recently shown by Twenhofel,^ who, however, 
Fig. 2. — Sketch showing form and distribution of the Aspidella-like markings on a portion of 
the rock. X i- 
states that the slope of the sides of the depression formed 
by a bursting bubble depends upon the viscosity of the mud 
and its angle of repose. Gas seeping up through mud of very 
low viscosity and escaping in a succession of very small 
bubbles would not form a deep crater, but the disturbance of the 
mud would rather be evenly distributed around the vent. In 
our specimens, as in some of those from Newfoundland, the 
markings are practically flat. It is noteworthy that in the 
material from Newfoundland the markings do not varj' much 
in relative depth on any one surface, although there is a 
great deal of variation in different pieces of slate. If Aspidella 
is a fossil we should have to explain why the parent organism 
1 Twenhofel, W. H. Impressions made by bubbles, raindrops and other 
agencies. Bull. Geol. Soc. Amer., vol. 32, p. 359-371, 1921. 
