484 J. J. Harris Teall — The Lizard Oabbros. 



I have not as yet been able to find a saussurite in tbe Lizard 

 District answering to Cathrein's description. 1 



The felspars of certain gabbros in which the development of 

 saussurite has not taken place at all, or has only occurred to a very 

 limited extent, often give evidence of having been affected by pro- 

 found mechanical disturbances. The twin lamella? are frequently 

 bent and, when the limit of elasticity has been exceeded, the crystal 

 grains have been fractured. The extinction is not sharp and definite. 

 Dark shadows sweep across the sections as the stage is rotated. In 

 slides exhibiting these characters large felspars are often seen to 

 break up, in certain places, into aggregates of minute, colourless, and 

 for the most part simple grains. Such aggregates must certainly 

 be regarded as of secondary origin, for they are entirely unknown in 

 the unaltered felspar-diallage gabbros and frequently contain needles 

 of actinolite, a mineral undoubtedly of secondary origin. They 

 correspond to the felspar-mosaic of Lossen. 2 



The pyroxene in the least altered rocks is a pale green diopside. 

 In microscopic sections it is almost colourless. Cross sections show 

 an optic axis in convergent polarized light and frequently the two 

 pinacoidal as well as the two prismatic cleavages. Longitudinal 

 sections give a maximum extinction of about 40°. The mineral is 

 therefore a monoclinic pyroxene. In addition to the cleavages above 

 mentioned, we find occasionally incipient stages of diallagic lamina- 

 tion. This lamination is not as a rule persistent throughout the 

 crystal, but limited to the neighbourhood of cracks. In the ordinary 

 gabbros the pyroxene is a true diallage with a decided bronzy lustre 

 on the planes of easy separation, which planes run parallel with the 

 orthopinacoid. In some of the coarse-grained rocks the diallage 

 crystals often measure two or three inches across. 



The change of pyroxene into hornblende has been described and 

 illustrated by Prof. Bonney. It is a common feature in the Lizard 

 gabbros and often accompanies the change of felspar to saussurite. 

 Many different varieties of secondary hornblende may be observed 

 depending on variations in colour and habit. Sometimes the horn- 

 blende occurs in the form of homogeneous crystalline grains with 

 strongly marked cleavages. This variety may be termed compact 

 hornblende. At other times the hornblende is distinctly fibrous, 

 and at others it occurs in the form of needles. The two latter 

 varieties are generally termed uralitic and actinolitic respectively. 

 Actinolitic hornblende frequently forms radiating fringes round the 



1 A saussurite composed of zoisite and albite occurs in Sangomore Bay near 

 Durness, Sutherlandshire. Under the microscope this is seen to consist of minute 

 colourless prisms embedded in a colourless felspathic matrix. The prisms possess a 

 high refractive index, weak double refraction, cross- jointing at intervals, and give 

 straight extinction. Small fragments of the felspar with which they are associated 

 may occasionally be obtained and when tested in the Bunsen's burner give the flame 

 colouration and fusibility of albite. In the mass this saussurite is white or a very 

 pale pink. It contains hornblende and is associated with a felspathic gabbro in 

 which the diallage has been converted into uralite and actinolite. 



2 Studien an metamorphischen Eruptiv- und Sedimentgesteinen, Jahrbuch d. k. 

 preuss. geol. Landesanstalt fur 1883 und 1884. 



