26 



D. P. PENHALLOW ON 



1 



Section through growth-ring of Larix 



occidentalis showing character 



of radial fracture x 50. 



A more detailed examinatitm of such fractures shows 

 very clearly that the line of rui)tiire passes tlirough the 

 median plane of the primary cell wall, as is manifest in 

 the following iigure, which shows the position (a) of tig. 1 

 more highly magnified. From this it liecomes evident 

 that the cohesive power of the primary cell-wall is low, and 

 it is now not a question of structural weakness, but one of 

 organic weakness. This [loint will become clearer when 

 we recall the fact that the primary cell-wall is not one 

 homogeneous membrane but is made up of the two mem- 

 l)rancs of contiguous cells which liave become united in 

 development. We may thus express tlie facts observed by- 

 saying that the cohesive power is weak by reason of certain 

 deficiencies in the cementing material which unites the 

 two layers of the primary cell-wall. Under ordinary 

 circumstances this cohesive power exceeds the strength of 

 the medullary rays, and these latter structures, being lines 

 of least resistance, are the ones through which radial 

 fracture is first established ; but here the case is otherwise. 

 That all coarse-grained Douglas fir should exhibit this 

 peculiarity in a more or less marked manner, while the 

 fine-grained varieties do not ; that our Kootanie specimens 

 oi Larix should also show it, while the same species from 

 other localities do not, at least in so marked a degree, 

 seems to indicate that under peculiar conditions of growth, 

 whether of soil, climate or exposure, one or all, certain 

 molecular alterations are etïected in the organization of the 

 primary cell wall, whereby it loses in cohesive power, and 

 its strength, relatively to that of the thick secondary wall, 

 or even to the medullary rays, is very deficient. The 

 precise cause of this deficiency does not appear from our 

 present examination, but will be dealt with in another con- 

 nection. 



(b) The rods composing the Kootanie coal have been 

 shown to have a diameter ranging from 0-5 mm. to 2 mm., 

 while there is a uniform absence of any superficial markings 

 as representing the impress of surrounding structure. Since our original examinations were 

 made, however, we have received from Mr. W. S. Gresley of Erie, Pennsylvania, specimens 

 of similar coals from the Pittsburg beds, and he now reports others of the same kind as 

 occurring in Carbon county, Wyoming, and in Illinois. It thus apjiears that these coals 

 are of widely distributed occurrence. In the case of the Wyoming coal Mr. Gresley reports, 

 as a result of his examinations,' that many ot the rods have pale, anil>cr or milk-white 

 interiors, and upon combustion leave an ash in the form of a pale-brownish scale. In the 



Section of Larix occidentalis at a of 

 fig. 1, showing, the nature of the 

 fracture in detail, cm the pri- 

 mary cell wall. X 210. 



1 " American Geologist," x., 332. 



