I 



ORIGINAL ARTICLES. 



* 



I. — Foliation and Metamorphism in Rocks. 



By Professor T. G. Bonney, Sc.D., LL.D., F.B.S. 



(Concluded from p. 203.) 



N pressure-modified gneisses and schists certain minerals are often 

 distinctly secondary in formation ; actinolitic hornblende replaces 

 ordinary hornblende in a gneiss, as on the southern side of the 

 St. Gotthard Pass (31), or in a dark mica-schist, as in the Binnenthal 

 (32), 'or a tremolite appears in marble near the Campolungo Pass (33). 

 A mixture of crushed hornblende and felspar gives rise to a biotite, 

 small flakes of which may be built, like bricks, into a newly-formed 

 large crystal of hornblende, especially towards its exterior (34). 

 Glaucophane in hornblendic rocks, such as diorites and eclogites, is 

 often a secondary mineral (35), some constituents derived from 

 a crushed soda-felspar having combined with those of the original 

 hornblende. Rather large biotites have formed in a dark mica-schist 

 in the Binnenthal, and here also some of them have been developed 

 at right angles to the main pressure, but others in the direction of 

 it (36). Scales of chloritoid, sometimes a third of an inch in diameter, 

 are secondary formations in some gneisses and chloritic schists, and 

 kyanites in some micaceous schists suggest a possible reconstruction 

 of an original mineral. Chlorite itself is a mineral of secondary 

 origin, and such rocks as smaragdite-euphotide (37), saussuritic 

 gabbro, and possibly even ordinary gabbro ' are all more or less 

 altered from their original condition, and in some at least of these 

 cases pressure may have been a factor in the change. Besides this, 

 new felspars may be formed in rather basic igneous rocks which have 

 been much crushed, such as the griiner Schiefer of the Alpine and 

 other regions. 2 In most of these cases pressure has been essential as 

 a preliminary factor (i.e. the rock must have been more or less 

 crushed), but how far it has operated in the "rebuilding" process is 

 less easily determined. Water has probably aided, and there may 

 have been some, though not a great, elevation of temperature. 



We can also find instances of intermediate or partial metamorphism, 

 i.e. rocks in which an original fragmental structure has not been 

 wholly obliterated. Of this the noted section at Obermittweida 

 affords a striking example (39). Here a mass of gneiss, very probably 

 of igneous origin but subsequently affected by pressure, is overlain 

 by conglomerates and other sedimentary rocks. The matrix of the 

 one and the materials of the other are sufficiently metamorphosed to 



1 Diallage may be always a mineral of secondary origin ; at any rate, we can 

 detect under the microscope grains of augite partly converted into it, and 

 discover that the hornblende in not a few diorites was once an augite ; but 

 on the history of hornblendic gabbros it is needless to dwell (38). 



2 These, as it has been proved, are plagioclase, but with a higher percentage 

 of silica and soda than the original mineral : albite or oligoclase, with a slightly 

 porphyritic habit, forming in a rock of which labradorite was a constituent. 

 These, it may be added, often fail to exhibit the characteristic oscillatory 

 twinning. 



