HEMITHRENES AND OTHER CALCITIC ROCKS. 53 



par MM. Scheerer et Keilhau"*. He also mentions that at 

 Laveline " le calcaire passe insensiblement au gneiss encaissant/ 



The conclusion we have formed respecting the origin of these 

 tongue-shaped masses of hemithrene will be obvious to the 

 reader. Undoubtedly there is a decided difference between 

 them and the enclosing gneiss ; it cannot, however, have been 

 caused by any mechanical break. Nor can the hemithrene be 

 infolded masses due to a crumpling of the beds ; for the gneiss 

 is plainly seen passing into the hemithrene both vertically and 

 horizontally, the change of colour (dark in the former and light 

 in the latter) affording the clearest evidence of the transition, 

 while the bedding is perfectly even and only slightly disturbed. 

 Even hand specimens may be collected with the two rocks in 

 such union as to make it impossible to detect any thing like a 

 mechanical division between them. 



With these evidences before us, and taking into consideration 

 various others that have been adduced, including those revealed 

 by the Cleggan dyke, proving rock-forming siliceous minerals 

 to have had their calcium silicate changed into a carbonate, 

 the conclusion becomes inevitable that the hemithrene of St. 

 Philippe is a methylosed product after gneiss. 



M. Rozel held the opinion that the hemithrene masses of 

 the Vosges are veins of igneous origin f thus agreeing with 

 Emmons (1842), Mather and Von Leonhard (1833), with respect 

 to corresponding cases made known by each as occurring in 

 other regions. Delesse, on the contrary, conceives that the 

 Vosgesian masses are genetically contemporaneous with the 

 associated gneiss. Our view is that they are due to heated 

 water holding carbonic acid or carbonate of lime in solution, 

 which has penetrated the gneiss through joints and other open- 

 ings, and converted it, where so affected, into dyke-like masses 

 of calcareous marble {. 



Having shown the convertibility of mineral silicates into 

 calcite and calcitic rocks, the question that next turns up for 

 consideration is whether a similar change has taken place over 

 a large area. 



* Annales des Mines, 4 C se"r. vol. xx. p. 181. 



t Bull. Soc. G6ol. de France, vol. iii. pp. 215-235. 



{ It is said that the rock at Chippal resembles in its purity Pares marble. 

 I was unfortunately prevented reaching this locality during my visit to the 

 Vosges. W. K. 



