138 MINERALOGY AND LITHOLOGY. 



cussed in other parts of this report. It is the object of this chapter to 

 supplement the work in the field with the results obtained in the labora- 

 tory. It aims clearly to explain the composition of specimens of rocks 

 which have been selected as typical, and to discover as much as possible 

 of the origin, mode of formation, and history of the masses, by the study 

 of samples. Microscopic study has of late been far the most fruitful in 

 the growth of the science, and therefore this method has been chiefly 

 employed. The value of work of this kind in connection with geological 

 surveys has been sufficiently well demonstrated by the labors of others; 

 and if this work is uninteresting it is the fault of the writer, for our 

 rocks furnish a most beautiful series of objects for microscopic investi- 

 gation. 



The field is also comparatively new. Dr. F. Zirkel, of Leipzig, one of 

 the most eminent authorities upon microscopic lithology, has written a 

 very valuable and most beautifully illustrated treatise on the rocks col- 

 lected by the United States Fortieth Parallel Survey (C. King, in charge). 

 But those rocks belong largely to the newer formations ; and with the 

 exception of isolated specimens which have fallen into the hands of 

 lithologists, the microscopic study of our old crystalline rocks has been 

 but little prosecuted, and hence a systematic investigation of their 

 microscopic structures and the properties of their constituent miner- 

 als, opens a field which cannot be barren of interest. 



But the results which in the past have been achieved by other laborers 

 in other ways must not be lightly passed. The laborious chemical 

 researches of Dr. T. Sterry Hunt, and the deductions which he has 

 drawn from them, are familiar to all, and lose no value to us because 

 performed on allied rocks in other regions. Incidental to his geological 

 investigations, most valuable lithological conclusions have been obtained 

 by Prof. J. D. Dana ; and besides these gentlemen, a large number of 

 able geologists have studied, with greatest care, either parts of our forma- 

 tions, or others closely allied to them. If, now, in approaching this sub- 

 ject from a somewhat different standjooint, in many cases the same con- 

 clusions are reached, the author would wish to add them to the credit of 

 those gentlemen. He would also recognize the labors of the European 

 lithologists who have most carefully studied allied rocks by the same 

 methods here employed, and whose results constitute the larger part 



