1893.] THE MICROSCOPE. 85 



THE MICROSCOPE'S CONTRIBUTION TO THE PHYSI- 

 CAL HISTORY OF THE EARTH. 



Professor T. C. Bonney, in the Rede lectures, takes for his 

 text the progress in geological research which has been directly 

 due to the revelations of the microscope. By the side of the 

 epoch-making discoveries of Darwin and Wallace, Bunsen and 

 KirchofT, he places the work in a humbler and more limited 

 sphere, of Sorby, who, in 1856, first described the results of 

 microscopic investigations into the structure of minerals and 

 rocks. The method employed by Sorby was, strictly speak- 

 ing, not novel, for, years before, in 1827, William Nicoll, of 

 Edinburgh, had made sections of fossil wood sufficiently thin 

 for microscopical examination. The device, however, had not 

 been generally applied, and to Sorby is due the credit of first 

 pointing out its wide possibilities. 



Before telling the story of microscopical research into the 

 history of the earth's crust, the author indicates briefly the 

 mode in which the microscope is used in the examination of 

 minerals and rocks. Slices are first cut by the lapidary's 

 wheel thin enough for most of the minerals to become trans- 

 lucent, if not. transparent. These are then examined under a 

 microscope of special construction, furnished with Nicol's 

 prisms and other optical appliances. Upon the history of the 

 two main groups of rocks the microscope has thrown much 

 light. For the igneous rocks it has simplified the classifica- 

 tion and determined the mutual relations, while for the sedi- 

 mentary group it has shown the true nature of their constitu- 

 ents and pointed out the sources from which they were de- 

 rived. But it is in helping to elucidate the problem of the 

 metamorphic rocks, of which much less was known, that the 

 microscope has been of the most service. 



The author deals at length with this portion of his subject, 

 and shows how the microscope has assisted in the attempt to 

 determine the history and mutual relation of these rocks. 

 One of the most important results within the last few years 

 has been the demonstration that without exception these crys- 

 talline schists are very old, all probably older than the first 

 rocks in which traces of life have been found. The conclusion 

 at present arrived at is that " the environment necessary for 

 changing an ordinary sediment into a crystalline schist existed 

 generally only in the earliest ages, and but very rarely and 

 locally, if ever, since Palaeozoic time began." 



The crystalline schists, then, are the relics still preserved to 



