1S15.] Experiments upon Green Uran Mica. 281 



Of the Straight-Tabular Rent. 



This rent is straight in both directions. Hence its name. Its 

 position is always at riglit angles to that of the strata. Hence it is 

 generally either perpendicular, or angular with a great ratio of 

 angularity, or approaching nearly to a perpendicular line. There is 

 no alteration in tiie position of tlie strata contiguous to it, as any 

 stratum on one side is opposite its corresponding part on the other. 

 This is a circumstance that at first sight distinguishes it from the 

 bended-tabular rent. It is filled with matter that entered it from 

 above ; such as the matter of green-stone, basalt, &c. or clav, 

 stones, sand, and gravel. 



This rent was formed when the earth's matter was so near its 

 present state of solidity, that the inecpiality in its contraction was so 

 very small as not to be discernible. Hence we find the strata on 

 the same level on both sides of it. 



The cylindrical and ovalar rents I will not at present describe, as 

 they are not very important phenomena in a geognostic point of 

 view. They arc very well described in William's Mineral King- 

 dom, under the names of ))ipe and flat veins. 



in my next communication I will describe the junctions of 

 tabular rents one with another. 



Errata in Mr. Longmirt's fint Communicalion on Rails, 



Read as follows (he sentence which begi^is on the 42il line of the 84th page: • 



" And Professor Jameson nieiilions two near Freyberg, in Germany ; one of 

 which is more than four miles in its horizontal direction, and the other more than 

 ten miles in tliis direction." 



Tlie letter u- in fig. 1, Plate XXTX., ouglit to have been j^Iaced close to the 

 highest extremity of the rent, v, tc, h. 



Ill fig. '.i of tlie same plate .-.unie letters are misplaced : these, however, will be 

 best corrected by repeating the figure in a future plate. 



Article V. 



Experiments upon Green TJran Mica, with a view to its Chemical 

 Analysis. By the Rev. W. Gregor. 



Any general account of the external character and appearance of 

 a mineral that is so well known would he superfluous. A scientific 

 dci^cription, that can be useful or interiisting, must be left to the 

 experienced crystallographer. 



'I'he subject of the following observations was raised in the 

 copper-mine called Gunnis Lake, in the eastern extremity of the 

 county of Cornwall. 



The crystalline lamina; are, in general, very free from extraneous 

 iubstunccb. Small fraguiculs of quartz, however, and a iigla ochrey 



