151 



the Hartz diabases. The subject here referred to at considerable length will 

 be seen to be one of great importance when we have to discuss the origin of 

 hornblende-schists. 



It must not be supposed that all saussurites possess the composition and 

 structure of those which have especially engaged the attention of CATHREIN. 

 KLOOS (I) has described the gradual passage of a perfectly fresh violet 

 labradorite (Ab, An 2 ) into a milk-white opaque mass in the case of a gabbro 

 occurring at Ehrsberg, in the southern part of the Black Forest. 



Examined under the microscope the white mass was seen to consist of a 

 striated felspar and a mineral which occurred in irregularly bounded grains 

 without a trace of cleavage. In thin sections these grains gave bright tints 

 under crossed nicols. The felspar of the opaque white mass was found to be 

 an albite having the composition Ab lo An t and the indefinite minerals proved 

 to be a lime- zeolite of the scolecite t} T pe. The specific gravity of the mixture 

 was found to be 2*598. 



The secondary felspar was determined by chemical analysis and by 

 optical methods, so that no question can arise as to the accuracy of the result. If 

 we compare the work of KLOOS with that of CATHREIN we observe that the 

 principal difference lies in the fact that scolecite takes the place of zoisite, and 

 that the resulting mixture has consequently a lower specific gravity. 



Dr. REUSCH has described the microscopic structure of the saussurite of 

 a rock from the neighbourhood of Drontheim in Norway. (2) It consists of an 

 aggregate of small columnar crystals or rounded grains of epidotc. Plagioclase 

 is quite subordinate to the epidote. The epidotc is either perfectly colourless 

 or a pale yellowish green. The characteristic pleochroism can only be 

 recognised in thick sections. The refraction and double refraction are both 

 strong and the basal cleavage is well marked. The extinction referred to 

 this cleavage varies from to 28. One interesting feature in this epidote is 

 the almost constant presence of twinning, parallel to the orthopinacoid (100). 

 This twinning can only be recognised when the grains are near the position 

 of extinction, in consequence of the fact that the r/-axis of elasticity makes a 

 very small angle (less than 3) with the vertical axis of the crystal. 



The modifications which the felspars of basic igneous rocks undergo 

 when subjected to mechanical stresses have been studied by several observers, 

 and especially by WEKWEKE, LOSSEN and LEHMANN. The felspars of many 

 rocks of the gabbro-family are often seen to be in a state of strain. This is 

 proved by a curvature in the twin lamellae, and by the fact that the extinction 

 under crossed nicols is not sharp and definite. Dark shadows corresponding 

 to the radii of curvature sweep across the sections as the stage is rotated. 

 When the limit of elasticity has been exceeded the crystal is fractured, and in 

 this case it frequently happens that the twin lamellao terminate abruptly at 

 the cracks, or run out from them only for a short distance into the crystal- 

 substance. These facts, which have been described and figured by WERWEKE (3) 



(1) N.J. Beilage. Band III., 1884, p. 34. 



(2) N.J. 1883. Band II., p. 179. 



(3) N.J. 1883. Bnml II.. p. 07. 



