﻿E. S. Dana — Broohite from Magnet Cove, ArJc. 315 



large size, sometimes 1 to 2 inches in length ; here the pyramid 

 z (112, J) is alone present. This form, though so simple, is 

 interesting because of the similarity it bears to a common form 

 of rutile, the allied species, also Ti0 2 in composition. This 

 resemblance is apparent at once, as was remarked by vom Eath, 

 and is in a measure borne out by the angles of the two species. 

 In rutile the prism is one of 90° and the pyramid s (111, 1) is a 

 tetragonal pyramid with a terminal angle of 56° 52-J' ; in this 

 related orthorhombic form the prismatic angle is 80° 10' and 

 99° 50' and the pyramid z has terminal angles of 53° 48' and 

 44 46', calculated from the measurements by von Kokscharow 

 on the usual assumption that the species is orthorhombic. It 

 is interesting to note that this type of crystal is the one which 

 most frequently shows the paramorphic change to rutile. 

 Figure 4 represents a form much like that just alluded to but 

 showing also the common brachy-pyramid e (122, 1-2) which by 

 some authors is made the unit pyramid. Figures 2 and 3 show 

 other crystals much smaller, and marked by the presence of the 

 basal pinacoid. The cn 7 stals, represented in figures 5 to 8, 

 were small, about -J to J inch in length, and of a rich reddish 

 brown color, different from the common color of the crystals of 

 the locality, which is deep black. The pyramid which predomi- 

 nates here, sometimes to the obliteration of other terminal faces, 

 is the obtuse brachy-pyramid £ (124, -§-2) with terminal angles 

 of 48°54'=XX' // (124*134), and 28° 28'=// (124^124). The 

 planes z and £ are often striated deeply, parallel to their mutual 

 intersections, and the oscillatory combination of these planes 

 is sometimes so marked that the termination appears to be made 

 by two brachydomes. Figure 6 is a basal projection of a crys- 

 tal near that shown in fig. 5, but having also the planes c and e. 



The crystal drawn in figs. 7 and 8 is interesting as showing 

 the rare plane Q (234, j-J-) in the zones 2, e and £, m. This 

 plane has not been observed before at this locality, and has 

 only been noted by Groth and Bucking* on a crystal from the 

 Maderanerthal. Figure 16 shows another prismatic form with 

 an acute termination formed by the dome t and the pyramid 

 e. Figures 14 and 15 represent a short prismatic form of un- 

 usual complexity ; fig. 25 (after Pen field) is a basal projection 

 of an allied form, but one in which the pyramid £ is a narrow 

 bevelment of the brachy-diagonal terminal edge of 2, much as z 

 bevels the macro-edge of e. 



Figures 11 and 12 represent a rare type of form in which 

 with the unit prism we have also the prisms I (210, 1-2) and 

 ^(120, i-2). This last plane is a new one for the species; it 

 was determined by the measured angles : 



(j> / =l2O a 120 = 62° 1'; 61° 26' calculated (Kokscharow). 



* Mineralien-Samrnlung. Strassburg, p. 110. 1878. 



