184 Reviews — TJie Micrographic Dictionary. 



knowledge of strata, both altered and unaltered, and of igneous 

 rocks of various ages, different sources, and manifold conditions. 

 The special organisms, too, that occur in clays and sandstones, and 

 that make up tripoli, coal, and limestones, have to be fully worked 

 out, and explained by the structure of existing organisms. Many 

 an interesting point is waiting for elucidation — such as the modes 

 of origin of flint, hornstone, and chert in limestones ; the part 

 played by various plants and their several organs in supplying 

 matter for coals and lignites ; the nature and function of Coccoliths, 

 so widely spread and so long persistent, according to Huxley and 

 Guembel ; the discrimination of doubtful bones, whether of reptile, 

 bird, or mammal ; and many other objects — as Conodonts, Eozoon, 

 Stromatopora, G-raptolites, and fossil Sponges — are open to further 

 illustration under the microscope ; and the fossiliferous limestones 

 will supply material for a life of study to a microscopist, like the 

 great Ehrenberg, equal to- the elaboration of these wondrous 

 " halibioliths," or marine rocks of organic origin. He must be 

 patient and acute, neither jumping at hasty conclusions, nor caught 

 by partial analogies and false similitudes. He must have an 

 educated hand and eye, with the experience of others at his elbow. 

 The Introduction of the " Micrographic Dictionary " will go far to 

 instruct him how to observe, and the body of the work will prove a 

 faithful guide to the expositions already made of living and extinct 

 organisms, of much indeed of both animate and inanimate materials 

 that lie is likely to meet with in his researches. 



One of the most remarkable instances in which the microscope 

 has been successfully applied to the elucidation of rock-structure is 

 the study of crystals in granites and lavas, by Mr. H. C. Sorby, F.R.S., 

 in 1858; and the concluding words of his memoir^ on the subject 

 are so apt and truthful that we reproduce them as a wholesome 

 stimulus to microgeologists : 



" These results are all derived from the study of the microscopical 

 structure of the crystals ; but my own observations in the field lead 

 me to conclude that they agree equally well with the general struc- 

 ture of the mountains themselves, and serve to account for facts that 

 could not have been satisfactorily explained without the aid of the 

 microscope. And here I cannot but make a few remarks, in con- 

 clusion, on the value of that instrument, and of the most accurate 

 physics in the study of physical geology. Although with a first-rate 

 microscope, having an achromatic condenser, the structure of such 

 crystals and sections of rocks and minerals as I have prepared for 

 myself with very great care can be seen by good daylight as 

 distinctly as if visible to the naked eye, still some geologists, only 

 accustomed to examine large masses in the field, may perhaps be 

 disposed to question the value of the facts I have described, and to 

 think the objects so minute as to be quite beneath their notice, and 

 that all attempts at accurate calculation from such small data are 

 quite inadmissible. What other science, however, has prospered by 

 adopting such a creed ? What physiologist would think of ignoring 

 2 Quart. Journ. Geol. Soc, vol. xiv., p. 497. 



