MICEOSCOPIC STRUCTUEE 35 



body, is rarely developed among the constituents of the rock 

 itself. The columnar structure of many lavas and dike rocks 

 has already been alluded to: occasionally the mineral constitu- 

 ents of some secondary rocks are arranged after this manner. 

 A cavernous or cellular structure is developed through the re- 

 moval by solution of some constituent or the weathering out of a 

 fossil. As an original structure it occurs in many rocks of chem- 

 ical origin as the stalagmitic deposits in caves, travertines, etc. 



A laminated or banded structure, due to the arrangement of 

 the constituents in parallel layers or bands, is common in rocks 

 of sedimentary origin, particularly in sandstones and shales. 



2. Microscopic Structures. — Many, if not indeed the ma- 

 jority, of rocks are so fine grained and compact that little of their 

 mineral nature or structural features can be learned from exami- 

 nation by the unaided eye. This difficulty made itself apparent 

 very early in the history of geological science, and to it is per- 

 haps due, more than to any other single cause, the apparent 

 crudities and fallacies of the early workers. As long ago as 

 1663, the microscope had been to some extent utilized for the 

 examination of minerals; but its application to the study of 

 rocks remained long unrecognized, though early in the nineteenth 

 century Cordier and others utilized it in the study of rocks in 

 a pulverized condition. It was not until about 1850, when the 

 subject was taken up by H. Clifton Sorby of England, that the 

 possibility of studying rocks in thin sections under the micro- 

 scope began to be appreciated. Even then the idea failed to 

 bear its legitimate fruits until transplanted to German soils, 

 where, under the fostering care of Professor Zirkel of Leipzig, 

 it soon began to yield an abundant harvest; and to-day the 

 branch of the science of geology known as microscopical pe- 

 trography hold a prominent place in all the leading universities, 

 both domestic and foreign. The efficiency of the method is 

 based upon the fact that every crystallized mineral has cer- 

 tain definite optical properties ; i, e,, when cut in such a way as 

 to allow the light to pass through it, will act upon this light in 

 a manner sufficiently characteristic to enable one working with 

 an instrument combining the properties of a microscope and 

 stauroscope to ascertain at least to what crystalline system it 

 belongs, and in most cases by studying also the crystal outlines 

 and lines of cleavage the mineral species as well. To enter 

 upon a detailed description of the method by which this is done 



