4 z6 THE POPULAR SCIENCE MONTHLY. 



dictated by common-sense, with the object of establishing and main- 

 taining, in the words of the old maxim of the sanitarian, mens sana 

 in corpore sano. 



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EOCK-STRUCTURE. 



Br Kev. J. MAGENS MELLO, M. A., F. G. S. 



THE study of rock-structure is one of great interest to the geolo- 

 gist, and not only does it teach him the varkms materials of 

 which any particular rock is built up, but it will often lead him to the 

 knowledge of wonderful facts relating to its origin and past history, 

 and will enable him to trace some of the many changes to which it 

 may have been subjected during the lapse of time. 



I propose to illustrate this by taking some familiar specimen and 

 showing the ways in which we may investigate its nature and history. 



Suppose we take a piece of granite and see what we may learn 

 about it. There are few persons but are acquainted with this rock 

 in some one or more of the forms in which it is found. Our public 

 buildings often present us with splendid illustrations of granite, 

 sometimes roughly hewed, as it has come from the quarry ; in other 

 cases highly polished. We have seen the fine gray stones from Aber- 

 deen, or the beautiful red ones from Peterhead and elsewhere. Xow, 

 when we begin to examine a piece of one of these granites, we see at 

 once that it is not an homogeneous stone such, for instance, as is a 

 bit of flint but that it is built up of various dissimilar-looking mate- 

 rials ; and we may notice, moreover, that one or more of those mate- 

 rials is crystalline, that it is shaped in some regular geometrical form. 

 We shall probably be struck with certain whitish or flesh-colored 

 crystals, more conspicuously prominent than the other substances of 

 which the specimen is composed. With some care we may be able 

 to make out in part the form of these crystals, and perhaps to meas- 

 ure one or more of their angles ; then, too, we shall notice that these 

 crystals are apparently imbedded in a more glassy-looking substance 

 of a clear and grayish color, and here and there we shall observe 

 some bright spangles of a thin flaky mineral. We shall thus have 

 seen the three principal minerals of which typical granite rock is 

 composed; the larger opaque crystals, whether white or pink, are 

 feldspar, the glassy mineral is quartz, and the little glittering spangles 

 are mica. We may next proceed to a more detailed examination of 

 each of these in turn. We will first ask the chemist what he can tell 

 us of their composition. The chemist is not satisfied with merely 

 knowing that a certain mineral occurring in certain definite crystal- 

 line or other forms is quartz, another feldspar, and so on ; but he asks 

 further : " What is this quartz ? Is it a simple body, or is it, simple as 



