356 Mr. Mallet on the Physical Conditiom 



ness to brittleness being almost instantaneous ; but the results, and their explanation, are the 

 same as in Daniell's experiment. 



The chemical change, then, does not prevent this inversion and development of crys- 

 talline axis; on the contrary, if it induce the required condition of internal strain, it may 

 produce the same crystalline arrangement that heating, or cooling, or local pressure can do. 

 Nor will the general law, stated in the text, be disturbed by the circumstance of sudden 

 expansion taking place in a cooling, and therefore contracting solid, at the instant that it 

 assumes the solid form — as in bismuth, cast-iron, ice, &c. ; for expansion here is but the 

 same force as in contraction, changing all the signs; and, however the crystals may be 

 formed in the first instance, they are subject to the subsequent modification in axial 

 direction. 



Mineralogy and lithologic geology are full of examples of the play of these crystalline 

 forces under the influence of pressures due to gravity, or to change of temperature ; and 

 some of its obscurest phenomena are yet destined to receive light from the application to 

 them of the general law enunciated in the text. What geologist is there who has not 

 observed, that the integrant crystals, forming the mass of quartz and other such veins in 

 icrneous rocks, are all arranged in lines perpendicular to the bounding planes of the original 

 fissure — the lines of least pressure in the mass, as it was heated or cooled by the surrounding 

 rock? Upon a greater scale, we find the metamorphic crystals of changed rock, adjacent 

 to dykes of igneous rocks — as in the chalk penetrated by trap in Antrim, — stretching away 

 irom the walls of the dyke in lines perpendicular thereto ; and the arrangement of the trap, 

 so far as it is truly crystalline at the surfaces of contact, obeying the same law. In Soot- 

 land, coal converted naturally into coke, by intrusion of a trap-dyke, assumes the pseudo- 

 crystalline structure known of it, in planes of fracture perpendicular to the bounding planes. 



Perhaps even the yet unsolved mystery of the structure of columnar basalt may find 

 its key and solution, — not in this law, but by views which it shall suggest; as well as the 

 molecular conditions, upon the physical action of which the lamination and cleavage of 

 the slaty and other rocks of imperfectly homogeneous material has depended, the directions 

 of the pressures concerned in which Mr. Sharp and Mr. Sorby have developed with so 

 much ability. 



The distinction, however, is to be clearly observed between internal Tprcssure inducing 

 change of crystalline axes, in truly crystallizable solid masses, and pseudo-crystalline 

 arrangement, such as cleavage, lamination, &c., produced by pressure external to the mass 

 of an uncrystallizable solid, and the indispensable conditions for which seem to be hetero- 

 geneity of composition of the mass, and peculiarity of form in its particles. In nature, these 

 latter phenomena may be sometimes mixed up, more or less, with the former, where crys- 

 tallizable substances are diffused in a mass of uncrystallizable matter. 



The navigator in high latitudes has long been familiar with the dreaded fact, that the 



