34 EXTINCT MONSTERS 
York States—were made by the soft fragile larve of insects 
which existed at an early period in the world’s history. 
We now pass on to the consideration of certain impressions in 
the old Potsdam Sandstone of North America, which have been 
most carefully studied by the late Sir R. Owen. In the year 
1851, Logan exhibited before the Geological Society of London a 
small slab of sandstone showing some footprints, and a plaster 
cast from a longer surface of similar description. The original, 
weighing upwards of a ton, is in the Museum at Montreal con- 
nected with the McGill University The locality where it was 
found is on the left bank of the River St. Louis, at the village 
of Beauharnois, on the south side of the St. Lawrence, about 
twenty miles above the city of Montreal. Owen, in his first 
paper, came to the conclusion that the tracks were those of a 
tortoise. But further research caused him to alter this opinion. 
We only mention that to show how much care is required even 
on the part of the best naturalists to read the meanings of fossil 
tracks. A portion of the impressions now under consideration is 
shown in Fig. 1. They consist of a series of well-defined impres- 
sions continued in regular succession for four feet, and more; 
but only clearly for four feet. In this four feet there are thirty 
successive groups of footprints on each side of a furrow. The 
number of prints is not the same in each group. Where they are 
best marked, as in our figure, we see three prints in one group, 
two in the next and two in the third, followed by a repetition of 
the three prints (in our illustration each of the three groups is 
enclosed in an oval). These three groups (of 3-2-2 impressions) 
are distinctly repeated in succession along the whole series of 
tracks on both sides of the furrow. It will be noticed that in 
each pair of impressions the innermost pair are of equal size, but 
of the outer ones each is a little bigger than the last. An 
