1869.] Geology and Palseoniology. 117 



increase of confidence in, and appreciation of, Portland cement as a 

 budding material, and the consequent additional demand for it, has, 

 it appears, hitherto led to an insufficient care in its preparation, 

 calculated to bring its properties into distrust. The object of the 

 j)resent woi'k is primarily to remedy this evil, by instructing the 

 manufacturers in the best practice of the process in all its details, 

 and introducing into English manufactories the method at present 

 adopted in Germany in that branch of industry. Considering how 

 important a position good cement holds in almost all large Engi- 

 neering works, any additional information calculated to lead to an 

 improvement in its manufacture cannot but be accepted as a real 

 boon. It will, of course, always be advisable that Engineers should 

 sample, and carefully test, every delivery of cement employed upon 

 theii- works ; and it is only by such means, scrupulously carried out, 

 that its manufactm-e is likely to be kept up to a desirable state of 

 efiiciency. Increased demand for a manufactured article too often 

 leads to carelessness in its manufacture ; and, as we have already 

 said, it is only by persistent testings that a high standard is likely 

 to be efficiently maintained. The present work shows how the 

 manufactui'e of Portland cement may thus be successfully con- 

 ducted. 



7. GEOLOGY AND PxiL^ONTOLOGY. 



(Including the Proceedings of the Geological Society, and Notices 

 of Recent Geological Works.) 



In last quarter's Chronicle we briefly noticed the pubhcation in the 

 ' Philosophical Magazine ' of two instalments of Mr. J. Croll's paper 

 on Geological Time. Its completion in the November number of 

 that magazine enables us to give a resume of the arguments, and 

 the principal conclusions at which the author has arrived. 



The question, How far the variation of the eccentricity of the 

 earth's orbit may have brought about the great changes of climate 

 indicated by geological phenomena, has been often discussed, more 

 especially as regards the cause and date of glacial epochs. During 

 the past three millions of years, there have been three periods when 

 the eccentricity attained a high value. The first of these began 

 about 2,630,000 years ago, and terminated about 2,460,000 years 

 ago. The second began about 980,000 years ago, and terminated 

 about 720,000 years ago. The third began about 240,000 years 

 ago, and terminated about 80,000 years ago. The third period, Mr. 

 Croll considers, was the date of the Glacial epoch ; the second was 

 that of the Upper Miocene period ; while the third corresponded to 

 the Glacial epoch of the Middle Eocene period. Few geologists 



