84 T. S. Hunt on Granitic Rocks. 



define and classify compound rocks, it should be borne in mind 

 tbat tbej are not definite lithological species, but admixtures 

 of two or more mineralogical species, and can only be arbitra- 

 rily defined and limited. 



§ 3. Having thus defined the mineral composition of granitic 

 rocks, we proceed to notice their structure. Gneiss has the same 

 mineral elements as granite, but is distinguished by the more 

 or less stratified and parallel arrangement of its constituents, 

 and lithologists are aware that in certain varieties of gneiss, this 

 structure is scarcely evident except on a large scale ; so that tk 

 distinction between gneiss and granite rests rather on geognos- 

 tical than on lithological grounds. To the lithologist, in fact, 

 the granitoid gneisses are simply more or less stratiform gran- 

 ites, while it belongs to the geologist to consider whether this 

 structure has resulted from a sedimentary deposition, or from 

 the flowing of a semi-fluid heterogeneous mass giving rise to a 

 stratiform arrangement. 



§ 4. The rocks having the mineralogical composition of gran- 

 ites present a gradual passage from the coarse structure of or- 

 dinary micaceous, homblendic and binary granites to finely 

 granular and even impalpable mixtures of the constituent min- 

 erals, constituting the rocks known as felsite, eurite and petro- 

 silex. These rocks are ofi;en porphyritic from the presence of 

 and sometimes of crystals or grains of 



crystals of orthoclase, and sometimes of crystals or grains 

 quartz imbedded in the finely granular or impalpable pa 

 These felsites and felsite-porphyries are, in very many cases at 

 least, stratified or indigenous rocks, and they are sometimes 



granitic gneisses. The resemblances in ultimate composition 

 between felsites, granites and granitic gneisses are so close that 

 it cannot be doubted that their diff'erences are only structural. 

 § 5. Felsites and felsite-porphyries are well known in eastern 

 Massachusetts at Lynn, Saugus, Marblehead and Newburyport, 

 and may be traced from Machias and Eastport in Maine, along 

 the southern coast of New Brunswick to the head of the Bay 

 of Pundy, with great uniformity of type, though in every place 

 subject to considerable variations from a compact jasper-hke 

 rock to more or less coarsely granular varieties, all of which are 

 often porphyritic from feldspar crystals, and sometimes include 

 grains or crystals of quartz. The colors of these rocks are 

 generally some shade of red, varying from flesh-red to pun>le- 

 pale yellow, gray, greenish and even black varieties are how- 

 ever occasionally met with. These rocks are throughout tms 

 region distinctly stratified, and are closely associated with dio- 

 ritic, chloritic and epidotic strata. They apparently belong, 

 like these, to the great Hurouian system. 



