MICROSCOPIC PETROLOGY. 705 



imperfectly-preserved external forms of these fragments, was 

 called in question by some other Palaeontologists ; who thought it 

 more probable that these bones belonged to a large species of the 

 extinct genus Pterodaetyhts, a Flying Lizard whose wing was 

 extended upon a single immensely-prolonged digit. No species of 

 Pterodactyle, however, at all comparable to this in dimensions, 

 was at that time known ; and the characters furnished by the con- 

 figuration of the bones not being in any degree decisive, the question 

 would have long remained unsettled, had not an appeal been made 

 to the Microscopic test. This appeal was so decisive, by showing 

 that the minute structure of the Bone in question corresponded 

 exactly with that of Pterodactyle bone, and differed essentially 

 fr'>m that of every known Bird, that no one who placed much 

 reliance upon that evidence could entertain the slightest doubt on 

 the matter. By Prof. Owen, however, the validity of that evidence 

 was questioned, and the bone was still maintained to be that of a 

 Bird; until the question was finally set at rest, and the value of 

 the Microscopic test triumphantly confirmed, by the discovery of 

 undoubted Pterodactyle bones of corresponding and even of greater 

 dimensions, in the same and other Chalk quarries.* 



596. The application of the Microscope to Geology is not, how- 

 ever, limited to the determination or discovery of Organic structure; 

 for, as has been now satisfactorily demonstrated, very important 

 information may be acquired by its means respecting the Mineral 

 composition of Rocks, and the mode of their formation. "As long," 

 says Mr. David Forbes, "f "as the Geologist encounters in the field 

 only rocks of so coarse and simple a structure as to admit of being 

 resolved by the naked eye into their constituent Mineral species, 

 or of distinguishing the fragments of previously existing rocks of 

 which they have been built up, he may speculate with a fair chance 

 of success as to their probable origin or mode of formation. When, 

 however, as is more often the rule than the exception, Rocks are 

 everywhere met with presenting so fine-grained and apparently 

 homogeneous a texture as to defy such attempts at ocular analysis, 

 all speculations as to their nature and formation based merely upon 

 observation in the field, can but be compared to groping in the 

 dark, with the faint hope of stumbling upon the truth. In these 

 cases the Geologist must call in the aid of Chemistry and the 

 Microscope ; by Chemical analysis he learns the per-centage com- 

 position of the rock in question; whilst the Microscopic examina- 

 tion informs him how the Chemical components are Mineralogically 

 combined, and at the same time affords valuable information as to 

 the physical structure and arrangement of the components of the 



* See Prof. Owen's Monograph on "The British Fossil Reptiles of the 

 Chalk Formation" (.published by the Palaiontographical Society), p. 80, 

 et seq. 



t ' The Microscope in Geology,' in the " Popular Science Review, 

 October, 1867. 



