616 ME. H. H. AENOLD-BEMEOSE OH THE MICEOSCOPICAL [NoV. 1 894, 



blue and green of the first order and green of the second : the slight 

 traces of cleavage, and the fibres, where present, are parallel to the 

 length of the crystal. It follows that the pseudomorphs possess 

 slight cleavage in planes parallel to the macropinacoid of the 

 olivine. 



The Potluck and Peak Forest varieties of pseudomorph are 

 also found in outcrop 39, Nealow Lane, near Matlock. Both the 

 ophitic and granular augite occur. The felspars and augite are 

 generally fresh. I have collected specimens of rock of both kinds 

 of structure which can be arranged in two series, each being similar 

 to those of the Peak Forest rock above described. The most altered 

 in each series give cleavage-flakes like those from Potluck, the 

 olivine being replaced by the mica-like mineral, and the least 

 altered contain nuclei of fresh olivine surrounded by the Peak 

 Forest variety of pseudomorph. In one thin section of the ophitic 

 rock serpentine also occurs in the olivine-cracks. In the same 

 section we have the Peak Forest pseudomorph. In the freshest 

 specimen there is none of the dichroic mineral, and the olivine is 

 altered to serpentine along the cracks. The question we have to 

 discuss is whether the Potluck pseudomorph is a replacement of 

 olivine by biotite or some mineral similar to it, or by the mineral 

 called iddingsite, or by some other mineral. 



Prof. A. Penard l describes the replacement of olivine by mica in 

 the rocks of Platform Island. The pseudomorphs have the colours, 

 absorption, and extinction of biotite, and sections parallel to the 

 cleavage exhibit a black cross with negative double refraction. The 

 replacement of olivine by leaves of biotite is described by J. J. 

 Sederholm. 2 M. Schuster 3 describes the partial replacement of 

 olivine by biotite in an anorthite-gabbro from Pirchville, California. 

 H. von Foullon 4 describes the replacement of olivine by biotite in a 

 melaphyre. The only proofs he adduces are its colour, dichroism, 

 and parallel fibres. Nuclei of olivine often remain unaltered. 



Rosenbusch 5 describes pseudomorphs of olivine with leaves parallel 

 to (010) very pleochroic, the greatest absorption being parallel to 

 the leaves. The colour is between iron glance and pseudo-brookite. 

 He says that it has not been proved whether it is a red colouring of 

 olivine with clearer laminations parallel to (010) or a new sub- 

 stance. Dana 6 states that olivine sometimes becomes brownish or 

 reddish-brown and iridescent. It also splits into thin laminse as 

 the change goes on, sometimes so as to resemble a mica. Prof. 

 Iddings describes the alteration of olivine 7 to a fibrous material 



1 Eeport of ' Challenger ' Expedition, vol. ii. (1889) pt. vii., Petrology of 

 Oceanic Islands. 



2 ' Studien liber archaische Eruptivgesteine aus dem siidwestlichen Finnland,' 

 Tscherm. Min. u. Petr. Mitth. vol. xii. (1891) p. 106. 



8 Neues Jahrb. Beilage Band v. (1887) p. 520. 



4 ' Ueber Eruptivgesteine von Recoaro,' Tscherm. Min. u. Petr. Mitth. vol. ii. 

 (1880) p. 481. 



5 ' Mikroskopische Physiographie,' vol. ii. (1887) p. 489. 



6 ' System of Mineralogy,' 1875, p. 258. 



7 'Geology of the Eureka District,' U.S. Geol. Surv. Mon. vol. xx. (1892) 

 pp. 388-390. 



