324 RECORD OP CURRENT RESEARCHES RELATING TO 



tlie formed constituents of the cell-body ; and it is equally suitable 

 for determining the relative degree of diathermasy of the cell-contents 

 and the cell-membrane. In this way also the effects of higher degrees 

 of heat can be more conveniently brought into view than by aid of 

 heated object-plates. Lastly, it is self-evident that the method is 

 applicable for animals and animal tissues as well as for plants ; and 

 with it we can at the same time demonstrate the sensation of heat in the 

 lowest classes of animals (Protozoa and Coelenterata), and in certain 

 cases ascertain the truth respecting the presence and the seat of the 

 perception of light. 



Microscopic Examination of Sands and Clays. — Dr. Sorby's 

 Annual Address at the Geological Society on February 20th, dealt 

 with the application of the Microscope (in most cases with polarized 

 light) to the study of the formations of some of the sands and clays 

 with a view to see how much they could be found to tell of their 

 own history. 



The greater part of the material of the non-calcareous stratified 

 rocks must be traced to the mechanical breaking-up or chemical 

 decomposition of igneous or metamorphic rocks, though some of 

 the more recent may be derived directly from analogous rocks of 

 the early periods. The method of this breaking-up or decomposi- 

 tion was described, and it was pointed out that the change from 

 the temperature and conditions under which igneous rocks had been 

 formed to subsequent conditions had immense influence in these 

 changes. The plu tonic and metamorphic rocks, again, formed under 

 the influence of water, were so formed at a high temperature, and in 

 subsequent times water at a low temperature, especially when highly 

 charged with carbonic acid, has a disintegrating effect upon them. 

 When igneous and metamorphic rocks are weathered and broken up, 

 sand and fine mud result, composed not only of the original materials, 

 but also of products of chemical changes. In describing some of the 

 more important materials derived from older rocks, quartz was first 

 mentioned. It is found that the optical characters, the internal 

 structure, and the shape of some of the grains are so characteristic, 

 that they may teach their whole history and throw much light on the 

 true character of associated materials, though many are worthless for 

 such purposes. Other sands, besides quartz sands, and also clays, 

 have been studied in detail. Among the many interesting results, it 

 is found that micaceous sand, previously supposed to originate from 

 disintegrated granite, may come from felsite, as in felsite the Micro- 

 scope reveals flakes of mica the ^xr J^to^ of an inch in thickness. 



Considerable additional light is also thrown by Dr. Sorby's methods 

 on the recognition of volcanic glass and ashes in a deposit. The 

 glass has no depolarizing effect on polarized light, and in many cases 

 the characteristic vesicular structure can be seen. The external forms 

 of the fragments are also characteristic, while quite unmistakable is 

 the appearance resulting from the melted glass being blown out by the 

 internal evolution of gas. Fragments of pumice can be easily recog- 

 nized from the fact that it is made up of cells with more or less curved 

 walls of glass, sometimes only the to ^5^0 of an inch in thickness. 



