J. J. Harris Tea 11 — Hornblende-Bearing Rocks. 347 



Uamh, south of Stronchrubie, at a height of about 700 feet above 

 the sea. 



Characters of the Eock-Forming Minerals. 



Hornblende. — As this is the mineral which especially characterizes 

 the rocks in question, it will be well to describe it in the first place. 

 It must be remembered, however, that this does not imply that horn- 

 blende is always the most abundant mineral. In almost all cases it 

 occurs in well-formed crystals. The larger ones are found in the 

 rock associated with the quartzite. They form short stout prisms, 

 which, however, so far as my observations go, are not terminated by 

 clinodomes (Oil) as in Dr. Heddle's figure, but by the positive hemi- 

 pyramid (111) and the basal plane (001). The forms in the prismatic 

 zone are (110) and (010). The smaller crystals occur usually in 

 elongated prisms, and can only be studied in a satisfactory manner by 

 the use of the microscope. Cross-sections sometimes show the 

 ortho- (100) as well as the clino-pinacoid (010). 



The larger crystals may sometimes be completely detached from 

 the rock-mass, and are of such a size that their angles may be measured 

 with a contact goniometer. In all cases the hornblende is green 

 when examined in thin sections. The pleochroism is fairly strong ; 

 a = pale-green or yellowish-green, /3 and 7 = deep-green, with 

 only slight differences. One of the most interesting features is the 

 presence, in certain varieties of the rock, of a marked zonal banding, 

 due to a variation in depth of tint in the successive zones. In some 

 crystals a very large number of zones may be detected ; in others 

 only two, and in others the crystal appears homogeneous. All these 

 variations may be seen in one and the same rock-section ; a fact 

 which seems to show that the crystals have not been simultaneously 

 developed. The zones are in all cases parallel with the external 

 boundaries of the hornblende, except where an original crystal has 

 been obviously fractured and a fragment only is present. 



Twinning is frequently present, and the most common type is that 

 in which the orthopinacoid (100) is both twin-plane and face of 

 composition. Sometimes repeated twinning on this plan occurs, so 

 that a crystal is made up of three or four lamella?. 



The alteration of the hornblende is accompanied by the formation 

 of chlorite, epidote, and opacite. In some of the more altered rocks 

 the hornblende is entirely represented by a green chloritic aggregate, 

 in which specks of opacite are abundantly scattered. In these cases 

 the form of the original hornblende is perfectly preserved. In other 

 cases gi-anular epidote is associated with chlorite, and in others 

 epidote occurs to the exclusion of chlorite. In the last-mentioned 

 cases the original form is often lost. 



Felspar. — -In some of the rocks two generations of felspar may be 

 recognized. The larger porphyritic felspars possess sharply-defined 

 crystalline outline. They occur in moderately thick tables, with con- 

 spicuous development of the clinopinacoid (010), and often show a 

 most beautiful zonal banding due to variation in the optical characters 



