Birmingham Natural History. Society. 481 



sand, lias much the appearance of a line of old Siand Dunes, formed 

 at the edge of a sea-margin. 



The "Lower Gyclas Clay," containing Cyclas cornea, appear* to 

 be a lacustrijie deposit. 



The "Upper Peat" has an average thickness of 12 feet, it contains 

 a soft blue silty clay, from which petroleum has been obtained. 



The Alluvial Clays contain ScroUcularia piperata abundantly in 

 some places, and Gyclas cornea in others. 



lasiFOiaTS j^isriD DPis,oc!E]:EnDXisrc3-s. 



The Mdokoscopigal Examination of Eocks anb Minerals.^ 

 By Samuel Allpokt, F.G.S. 



THE application of the microscope to Geology is a subject which 

 has, I believe, been more generally neglected by men of science 

 than any other of equal importance, although it is easy to show that 

 this method of examination affords the readiest means of obtaining 

 information on many important points, and is, in fact, the only way 

 of ascertaining with certainty the composition of many compact or 

 fine-grained rocks. 



A valuable paper by Mr. Sorby, " On the Microscopical Structure 

 of Ctystals,"^ and another by Mr. David Forbes, " On the Micros- 

 cope in Geology," in the "Popular Science Eeview,"^ constitute, I 

 believe, the extent of the published information on the subject. No 

 branch of science presents a more promising field of inquiry, for the 

 nomenclature and classification of igneous rocks is in a lamentable 

 state of confusioUj and unfortunately the maps of the Geological 

 Survey of England and Wales tend rather to increase the difficulty 

 by the indiscriminate use of terms not very definite in themselves. 



It is evident that no satisfactory classification of rocks can be 

 made till their mineral constituents are accurately ascertained ; and 

 in the case of fine-grained or compact rocks, this cannot be done 

 either by chemical analysis or other ordinary methods of observa- 

 tion. In such cases, various expedients have been adopted, but only 

 approximative results were obtained, and that they were altogether 

 unsatisfactory will be evident from the following remarks of Bischof.* 

 "It appears," he says, "to be quite in vain to attempt a classification 

 of Greenstones; they constitute a series, the extreme numbers of 

 which arCj on the one hand, Augitic Porphyry, or Melaphyr, with 

 distinguishable Augite, and Labradorite ; and, on the other, Diorite 

 or Syenite, with distiuguishable Hornblende. Between these opposite 

 extremea there are many intermediate varieties, with regard to which 

 it is quite uncertain whether they are to be classed with augitic or 

 homblendic rocks." Again, Cotta® describes Aphanite as " Trap in 

 part, Melaphyr in part, a compact apparently homogeneous mass. 

 The separate ingredients of this rock are not to be recognized by the 



1 Proceedings of the Birmingham Natural History and Microscopical Society. 



'^ Quart. Journ. GeoL See, vol. xiv. ^ Vol. vi., p. 355 



* " Chemical Geology," -vol. iii., p. 300. * " Eocks Classified," g. 157,. 



