38 Mr. Brooke on the [July, 



These, however, are by no means so numerous or striking as 

 those afforded by the greywacke of North Devon, a circum- 

 stance apparently adverse to the theory which would attribute 

 these singular configurations to the agency of heat ; for we might 

 certainly expect that the killas, which is easily affected by that 

 agent, near as it is to the central granite, and traversed in all 

 directions by various dykes and veins, would have exhibited 

 more frequent traces of this nature than the refractory and 

 unbending sandstones. But this is a question of mere hypo- 

 thesis. This portion of the inferior slate does not (so far as my 

 knowledge extends) contain any imbedded minerals; near 

 Camelford, and at some other spots, 1 have observed in it small 

 contemporaneous veins or nests (vugs, as they are provincially 

 termed) of crystaUized felspar and chlorite. Most of its varieties 

 are readily fusible. 



iTobe continued,) 



Article IX. 



On the Crystalline Forms of Artificial Salts. 

 By H. J. Brooke, Esq. FRS. 



{Continued from vol. v. p. 452.) 



In my last communication I noticed the irregularity that 

 frequently occurs in the forms of crystals, whether natural or 

 produced by art, occasioned by an enlargement of some of the 

 planes, and a consequent comparative diminution of others. 

 This irregular character may be said to be almost general, and 

 very frequently might lead to an erroneous determination of the 

 true; forms of crystals, if we do not attend sufficiently to the 

 positions of their planes, to their cleavages, and to the measure- 

 ments of their angles. Another circumstance will also tend to 

 mislead us with regard to the forms of crystals, when compared 

 with the drawings by which they are represented : this is the 

 manner of their attachment to the mass to which they are 

 united ; sometimes they are attached by a lateral edge or plane 

 of the figure exhibited in the drawing, and sometimes by the 

 vpper summit ; in which latter case, the crystal would appear to 

 be inverted, and the order of the lateral planes of several of the 

 classes of prisms, when observed from left to right, would be 

 reversed. 



The measurement of corresponding planes on different crys- 

 tals will frequently differ more than half a degree, and may occa- 

 sion a difficulty in determining particular planes by measurement, 

 when they meet at nearly the same angle. The angles given here 

 are generally the mean of a considerable number of measurements. 



