J. J. Harris Teatt—The Lizard Gabbros. 483 



for a distance of about a quarter of a mile. A mass of precisely 

 similar gabbro occurs inland at G-winter. The two portions are 

 doubtless continuous, as represented on the geological map; and if so, 

 they form a dyke-like mass running inland in a N.W. direction. 

 The Karakclews gabbro is bounded on both sides by serpentine into 

 which it was intruded. Marked signs of disturbance occur near 

 both junctions and throughout the mass ; but, notwithstanding this, 

 veins of gabbro may occasionally be seen running out from the 

 larger mass into the surrounding rock. 



Another considerable exposure of gabbro occurs on the shore 

 north of Pen Voose, 1 near Landewednack. It is bounded on the 

 north by serpentine with gabbro veins, and on the south by a 

 remarkable group of rocks which Prof. Bonney terms " the granulitic 

 series." The gabbro is here traversed by veins of granite and mica- 

 diorite which will doubtless repay careful examination, but which 

 need not be further referred to on the present occasion. 



The principal mineralogical constituents of the Lizard gabbros are 

 plagioclase, augite or diallage, hornblende and saussurite. Olivine 

 is present in certain varieties, but does not appear to have been very 

 widely distributed in these rocks. In the least altered rocks the 

 plagioclase occurs in the form of grains of tolerably uniform size and 

 having nearly equal dimensions in the different directions. The 

 grains show broad lamellar twinning, and not seldom two sets of 

 lamellae are seen crossing each other at right angles. This is the 

 feature which is usually regarded as indicating simultaneous twinning 

 on the albite and pericline types. The extinction angles in thin 

 sections of the rock show that the felspar belongs to the labradorite- 

 anorthite series, but I have not as yet been able to make a more pre- 

 cise determination. In the more or less altered gabbros we see 

 various stages of the replacement of felspar by white or cream- 

 coloured saussurite. The first stage consists in the development of 

 white spots in the glassy felspar substance. These spots appear 

 snow-white under the microscope by reflected light, and opaque or 

 brown by transmitted light. They never exhibit any definite form 

 or cleavages. By the increase in the number and size of these white 

 spots and by other changes the felspar gradually passes into saus- 

 surite. This substance sometimes occurs in masses of considerable 

 size, and it is then remarkable for its toughness and hardness. Its 

 specific gravity is about 3 - l. Microscopic examination shows that 

 it is an aggregate and not a simple mineral, but it is by no means 

 easy to determine the precise character of the different constituents. 

 Cathrein's 2 investigations led him to the conclusion that saussurite is 

 an aggregate of zoisite or epidote and felspar, usually albite ; with 

 variable quantities of actinolite and tremolite as accessory constituents. 



1 There is some confusion about the naming of this locality. The small hay to 

 the north of 'which the gabbro is exposed is named the Balk on the one-inch map and 

 Parn Voose on the twenty-five inch map. The inhabitants of this district say ; that 

 this bay is called Pen Voose and that the place -where the life-boat is kept, otherwise 

 known as Church Cove, is Parn Voose. As the maps do not agree, I have folio-wed 

 the inhabitants. 



2 Ueber Saussurit, Zeit. f. Krystallographie, vol. vii. p. 234, 



