134 
POPULAR SCIENCE REVIEW. 
THE STRUCTURE AND ORIGIN OF LIMESTONES.* 
By H. C. SORBY, F.R.S., President of the Geological Society. 
I N his Anniversary Address, delivered at the annual meeting- 
of the Geological Society in February, the President, 
Mr. H. C. Sorby, F.R.S., departed somewhat from the ordinary 
form of Presidential addresses, and laid before the Fellows the 
result of some laborious original researches which seem to open 
up a new field of geological investigation, or at all events to 
suggest the application of a perfectly new method of research 
to the solution of questions which have certainly already, to 
a limited extent, attracted the attention of geologists. He 
devoted his address to the consideration of the structure and 
origin of limestones, relying mainly on his own observations, 
but incorporating general facts derived from other sources. 
Since, in order to properly understand the nature of the 
various constituent fragments of which many limestones are 
composed, it is necessary to know the organic and mineral con- 
stitution of the different living calcareous organisms, this ques- 
tion was first considered from a somewhat novel 'point of view y 
and very great attention was paid to their optical characters, 
by using a new form of apparatus, specially designed for such 
inquiries. The conclusion arrived at was that the microscopical 
structure depends on both organic and mineral growth, the 
influence of the one or the other varying greatly in different 
organisms. Much attention was also directed to their mineral 
constitution, so as to determine whether the carbonate of lime 
exists in the form of calcite or aragonite. The results are in 
some cases very curious and interesting, even from a biological 
point of view. Certain large natural groups are composed of 
calcite, and others of aragonite, whilst others are made up of 
well-marked layers of the two minerals. In a few cases they 
occur too intimately mixed to be separated. 
* We need hardly apologise to our readers for laying before them this 
abstract of a most interesting and important memoir. For all but the first 
paragraph we are indebted to the kindness of Mr. Sorby. 
