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 LX.VIII. Intelligence and Miscellaneous Articles. 



ORIGIN OF TRAP ROCKS. 



[The following is an abstract of a Paper on this subject by Mr. S. 

 Solly, read before the Royal Society on the 6th of March last.] 



WHILE Mr. Solly was examining the hills of porphyry near 

 Christiania, he discovered a low, detached, quadrangular 

 rock of trap of the species called diorite* which exhibited in a 

 manner that had never before been noticed, except in the experi- 

 ments of Gregory Watt, the gradual development of crystallization 

 commencing with globules of green pyroxene and white felspar, 

 sometimes enveloping each other, and varying in size from that of 

 a small pea to extreme minuteness ; they were thickly strewed on a 

 perpendicular side of the rock near the ground, and at a distance 

 had the appearance of cryptogamia. 



On a close examination he found some of the globules stretched 

 out, and in other parts close to the former; the felspar had shot into 

 parallel lines, which here and there had united to form rhombs. He 

 likewise found the pyroxene standing out upon the surface in re- 

 gular crystals, exhibiting a curious variety of modification 5 some of 

 them crossing each other like the double crystals of harmotome. 



To his communication of this fact, Mr. Solly has prefixed some 

 remarks on the probable history of rocks of trap, derived from a 

 comparison of the different situations in which he has met with 

 them. From observing their partial resemblance to lava, and that 

 they are most frequently met with in coal-fields, Mr. Solly infers that 

 they owe their origin to internal combustion ; and as little doubt 

 seems to remain that our beds of coal were formerly bogs, which 

 have been compressed and protected by a covering of clay and sand, 

 he conjectured that other masses of bog and combustible strata lying 

 more exposed have been converted into trap. The position of the 

 Swedish rocks, (from whose quadrangular form this name is derived,) 

 strengthened this conjecture, which was confirmed by his meeting 

 with a stone perfectly similar in all its characters, recently formed 

 by the spontaneous combustion of substances which perfectly cor- 

 respond with some of the strata* found beneath the Swedish rocks. 



Mr. Solly introduced the subject with observations on the assist- 

 ance geology ought to derive from the discoveries in experimental 

 science. Various observations on currents are interspersed through 

 this paper, to explain the difference of stratification in different si- 

 tuations. From the insulated character of the formations in coun- 

 tries intersected by granite, Mr. S. infers that the latter cannot be 

 a rock of posterior formation as the Scotch Plutonists imagine. 



An explanation is given of some instances of high temperature 

 in the lower part of deep mines, from which erroneous conclusions 

 have been drawn. Mr. S. touches on the subject of electrical as 



* They are sufficiently impregnated with bitumen and sulphur to be used 

 as fuel or roasted for alum. See Thomson's Travels. 



well 



