208 Prof. J. W. Spencer — High Continental Elevation. 



SiO, 62-86 



Al,6 3 20-1 



*Fe 2 3 2-6 



CaO 4-5 



K 2 0-36 



Ka 2 8-48 



H 2 1-3 



100-20 100-00 2-11 71-49 22-31 



3-3 1 



The constitution of the secondary felspar may be considered as 

 An x Ab 3 . 3 . The molecular rearrangement of the original labradorite 

 (Abx An 3 ) has resulted in the formation of a felspar allied to 

 oligoclase. We have accordingly the association of uralite and 

 oligoclase in the rocks, and it is hence interesting to recall G. Rose's 

 observations upon the subject. (Vide supra.) 



Owing to this alteration of the felspar, a large amount of lime 

 was set free, which induced the formation of epidote and zoisite. 

 The aggregates of these two minerals are sometimes found piercing 

 the bi-silicate in continuous lines, and thus preserve the ophitic 

 structure so characteristic of doleiites. 



Iron-pyrites and ilmenite are present : the latter is mostly altered 

 into granular aggregates of sphene. The amount of chlorite and 

 quartz is naturally in direct proportion to the degree of alteration the 

 rock has undergone. Apatite is rare. 



There is no evidence of the presence of either scapolite or sahlite 

 in the specimens examined. It is, however, highly probable that 

 both these minerals are to be met with in the greenstones of this 

 district, as Sir Charles Giesecke records their occurrence at the old 

 lead-mine in the neighbourhood of Strabane, Co. Tyrone. 1 The 

 scapolite shows a " four-sided prism" ; whilst the sahlite is described 

 under the old name "baikalite." The same mineralogist calls 

 attention to the presence of <; silver-white, pearly mica, resembling 

 lepidolite," at HolyhilL Co. Tyrone, where he also observed lievrite. 



14, Hume Street, Dublin. 



III. — The High Continental Elevation preceding the Pleisto- 

 cene Period (in America). 

 By Prof. J. W. Spencer, A.M., Ph.D., F.G.S., 



State Geologist of Georgia. 

 (Read before the Geol. Soc. Am., August, 1889.) 



IP, in the growth of the American continent, the moulding of the 

 land features had not largely depended upon its projection 

 above the sea, favouring or retarding the action of rains and rivers, 



* The Fe 2 3 was estimated by titration : the amount is certainly high ; hut the 

 sp.g. of the powder (2-645) shows that it cannot be ascribed to the presence of 

 epidote. 2-fi97 of Fe 3 3 would be equivalent to an admixture of about 5-87 of 

 epidote, which would demand a sp.g. of about 2-9 (cf. Cathrein, " Ueber Saussurit," 

 Zeits. f. Kryst. vii. 241-242). 



1 loc. cit. pp. 208 and 219. 



