OLDEE PEEIDOTITES OP SCOTLAITD. 411 



of an ultra-basic rock of a new and hitherto undescribed t}'pe. It 

 was originally a mica-picrite with strongly marked ophitic stnicture, 

 and exhibits evidence of some of its minerals having undergone 

 Schillerization ; but at the present time all the original minerals 

 are represented by their pseudomorphs — augite by hornblende, olivine 

 and enstatite by serpentine, and biotite by a curious hydrated 

 form of that mineral. 



The recognition of certain characters in the rock-forming minerals 

 as being original and essential, and the distinction of such from other 

 characters which are secondary and accidental, is of the highest 

 importance to the petrographer and geologist, and not less so to the 

 mineralogist. 



Rightly studied, these minerals are capable of furnishing 

 the geologist with evidence not only concerning the mode of 

 origin of the rocks of which they form a part, but also of the 

 changes which they have undergone since their first formation. The 

 study of the minerals included in the crystalline rocks is not less 

 important than that of fossils in the sedimentary rocks. And to 

 the mineralogist the study of the secondary characters of minerals, 

 and of the causes which have produced them, is equally necessary. 

 Eesearches of this kind, indeed, can scarcely fail in the end to 

 reduce many so-called mineral species to the rank of accidental, 

 though still highly interesting, varieties. But of still greater impor- 

 tance is the recognition of the fact that the investigation by the aid 

 of the microscope of the processes by which minerals have acquired 

 their several characters, and the consequent tracing of the evolution 

 of mineral species and varieties, is calculated to raise mineralogy 

 from its present rank as a merely classificatory science, to infuse it 

 with new life, to open out to it new realms of research, and to invest 

 it with a higher importance than is at present accorded to it in the 

 family of sciences. 



EXPLANATION OF PLATES X.-XIII. 



Plate X. 



[Note. — Some of the details of these figures can only be distinctly seen by 

 employing a low-powered magnifier.] 



Fig. 1. A crystal of felspar from a gabbro-vein traversing dunite at Scuir na 

 Gilean, Isle of Eum, viewed by polarized light with a magnifying- 

 power of 35 diameters. This crystal admirably illustrates the con- 

 clusion that the lamellar twinning must be regarded as a secondary 

 character of the plagioclase felspars. A large portion of the crystal 

 is quite free from any trace of the lamellar twinning. The crystal is 

 traversed by a number of cracks, and between the cracks, lamellar 

 twinning is seen in some cases to be dereloped. An examination of 

 the relations between the fissures and the lamellar twinning is con- 

 clusive as to the non-existence of many of the twin-structures before 

 the formation of some of the fissures which traverse the crystal. 

 This section also illustrates the manner in which lamellar twinning 

 is frequently developed along two different sets of planes in plagio- 

 clase felspar, and that these two sets of twinnings sometimes inter- 

 sect one another. The way in which these twin-lamellas are found 

 starting irregularly at certain points in the crj-stal, and dying away 



