Rev. O. Fisher—Secular Straining of the Earth. 275 
one of those figured by the writer in 1865 (see Quart. Journ. Geol. 
Soc. vol. xxi. pl. xiv. fig. 17) ; but the Wenlock specimen is broader 
in proportion to its length, and the keel of the valve is straighter, 
and the general outline more angular than is the case with the 
Ottawa specimen. ; 
I propose for this new form the trivial name of Canadensis, and 
trust that before long, through the researches of Mr. Ami, we may 
become aquainted with many more examples of the valves of this 
very interesting and widely-distributed fossil. 
TX.—Remarxs on Mr. Davison’s Parer on Sucunar STRAINING 
oF THE HARTH. 
By ,Rzev. O. Fisuer, M.A., F.G.S. 
VERY important discovery was made independently by Mr. 
Davison, and somewhat earlier by Mr. T. Mellard Reade, 
when they demonstrated that, on the hypothesis of the earth having 
cooled as a solid body, there is within it a certain level, which 
is called by Mr. Davison the “ surface of zero-strain,” and for which 
Mr. Reade has adopted the term ‘level of no strain.” At this level 
compression ceases, and for a certain distance beneath it extension, or 
stretching of the layers of the globe, takes the place of compression. 
There are one or two points in Mr. Davison’s paper which invite 
remark, because they have an important bearing upon the geological 
aspect of the subject. He states that he has come to the conclusion 
that, after an interval of 174,240,000 years since the earth solidified, 
the depth of the surface, or level, of no strain ‘‘was or more probably 
will be” five miles. But for geological purposes what we want to 
know is, what the depth of this level may be at the present time. 
Now, on the assumption that the earth has cooled as a solid, this is 
easily calculated, without the need of any knowledge either of the 
length of time since solidification, or of the conductivity of the 
materials of which the crust of the earth is composed. 
The formula which gives the depth of the level of no strain within 
a few hundred feet is simple. It is: 
6 temperature of ae es 
3°1416\ temperature gradient ry eee 
Depth of level of no strain = 
The temperature gradient at present is known to be +; of a 
degree Fahr. per foot. The only quantity to be guessed at is the 
temperature of solidification, which Mr. Davison, following Sir Wm. 
Thomson, puts at 7000° Fahr. The radius of the earth in feet is 
20,900,800. 
Working out the sum we then find that the depth of the level of 
no strain, on this hypothesis respecting the temperature, is at the 
present time 11,645 feet, or 2-2 miles. 
But 7000° Fahr. seems to be an excessive temperature for solidi- 
fication; and if we put it at 4000°, which seems more probable, the 
depth of this level at the present time would only be a little over 
half a mile. It must be remembered that the depth of this level 
