A CORRECTION: G. F. BECKER 
257 
cocci.^ For the purpose of comparison in this preliminary notice an 
illustration is here given of a group of recent forms as shown in the 
Encyclopedia Britannica^ and of the form shown in the cells in the thin 
sections cut from the fossil alga of the Newland limestone. 
4' 
♦ 
Figs, 2 and 3. Alicrococcus sp. undt. (X about 1100 diameters.) Average size of 
Micrococci 0.95 to 1.3 microns in diameter. (Slide D.) From locality 401b, Algonkian: 
Gallatin formation; north side of East Gallatin River, 5 miles (8 km.) east of Logan, 
Gallatin County, Montana. 
1 Walcott, Pre-Cambrian Algonkian Algal Flora, Smithsonian Misc. Coll., 64, No. 2 (1914). 
2 Idem, p. 116, pi. 23, fig. 1. 
^Encyclopaedia Brifannica, 11th ed., vol. 3, p. 160, fig. 5. 
A CORRECTION 
(Received March 29. 1915) 
In my paper on the earth considered as a heat engine, this volume, page 
81, the linear expansion of rock forming minerals was inadvertently taken 
at 100 times its real value. A square area of superficial rock of relatively low 
diffusivity would really need to be several hundred degrees hotter than the 
surrounding areas to be shattered by the compressive stresses called into play 
by mere difference of temperature. Similarly a mean temperature difference 
of 40° between oceanic and continental columns overlying the level of iso- 
static compensation, would by itself account for a difference of level of only 
about 39 metres. 
The error committed does not affect the general argument that the sub- 
continental shell acts as a heat engine, for it is known that several reversible 
processes such as elastic strain, expansion, liquefaction and volatilization are 
