154 CARNEGIE INSTITUTION OF WASHINGTON. 



rock. Moreover, the testing of the laboratory conclusions against 

 the results found in nature establishes the importance of this factor 

 in the producing of the great variety of igneous rocks, and enjoins 

 upon us the need of further study of its action. 



CALCIUM CARBONATE. 



With the same general purpose in mind, some consideration has been 

 given during the past year to one of the important sedimentary rocks. 

 Perhaps the most important single chemical substance found in the 

 sedimentary deposits is calcium carbonate, which occurs as large masses 

 of limestone and marble, also as chalk, coral, and in other forms ; more- 

 over, it is being deposited at the present time in many parts of the sea 

 and in many lakes. Indeed, one can hardly overestimate the useful- 

 ness to geological science of a thorough knowledge of the conditions 

 under which this substance can form, and of the quantitative impor- 

 tance of the various factors which influence its deposition. The 

 usefulness of such information is enhanced by the fact that it would 

 serve to elucidate a number of problems which at first sight bear no 

 apparent relation to the properties of calcium carbonate. 



An instance of such a problem is the proportion of carbonic acid in 

 the earth's atmosphere during its history, for a secular variation of 

 this proportion is very plausibly connected with great climatic changes 

 and especially with the appearance of glacial epochs. One can there- 

 fore readily appreciate the reason why calcium carbonate has already 

 formed the subject of a large number of papers, and yet, despite the 

 attention bestowed on it, there are some fundamental points which 

 are still obscure. It appeared probable that some of the outstanding 

 questions could be answered if the existing data were collated, ampli- 

 fied by further experiment where necessary, and interpreted by the 

 aid of the principles of modern chemistry. Such an investigation is 

 under way and the results are very promising. 



Perhaps the most important point which hitherto has received 

 inadequate attention (probably by reason of the apparently contra- 

 dictory character of the experimental data) is the signal effect of 

 changes in the proportion of carbonic acid in the air upon the amount 

 of calcium carbonate which a natural water will hold in solution. Thus 

 if the proportion of carbonic acid falls from 3.5 to 3.0 parts per 10,000 

 of air, the solubility of calcium carbonate in water in contact with 

 that air will fall from 66 to 63 parts per miUion. This decrease in the 

 proportion of carbonic acid over a shallow sea would be followed by 

 the deposition of about 2.5 tons of calcium carbonate per square mile 

 for each foot in depth of the water. Indeed, it is plausible that this 

 decrease in the free carbonic acid in the water — a decrease which may 

 be produced in many ways, by organic as well as by purely physical 

 agencies — is the immediate cause of the precipitation of a large part 



