116 FIELD GEOLOGY. 



have previously been regarded, and even analysed, as 

 perfectly homogeneous substances, envelop vast quanti- 

 ties of minute crystals of other minerals."* Here comes 

 in the use of the microscope ; by its means we also "learn 

 whether a rock, whose structure is too minute to be un- 

 derstood without it, is to be classed amongst the aqueous 

 or the igneous series," and we discover "minute or- 

 ganisms, such as foramenifera, diatomaceae, or faint 

 vegetable traces in rocks, in which, without its help, 

 nothing can be detected. The broken and often water- 

 worn fragments of the aqueous rock, derived it may be 

 in the first instance from the breaking up of igneous 

 rocks, will at once reveal its origin. 



" Igneous rocks are for the most part crystalline in 

 their structure, although we must at the same time re- 

 member that many crystalline rocks have been formed 

 directly from watery solution. Gypsum, calcite, rock- 

 salt, and some forms of quartz are examples of such, 

 but those that have been thus formed may be readily 

 distinguished. In his valuable paper ' On the Micro- 

 scopic Character of some Crystals,' Mr. Sorby calls par- 

 ticular attention to certain minute cavities, in even the 

 smallest crystals, which he shows to be the key to the 

 history of the crystal.''^ 



Those who have the opportunity of making for them- 

 selves microscopical examination of rocks will find much 

 valuable assistance in the monographs enumerated below 

 (p. 118), there being at the present time no work specially 

 devoted to the subject. 



* "Mineralogy." Rutley. 



t Microscopical Structure of Rocks. Mello. " Pop. Sc. Re- 

 view," Jan. 1875. 



