40 Reviews— Prof. Eehert — Older Rocks of N. W. France. 



Living entirely among the natives for many months, and speaking 

 their language, the author was able to test the power claimed by the 

 natives of being able to predict the outbursts of the geysers. He is 

 convinced that by constant observations on the direction of the wind 

 and the condition of the atmosphere, the natives have learnt to prog- 

 nosticate the movements in all these hot springs with wonderful 

 accuracy. He was also able to prove that during the whole tinie 

 of his lesidence in the district certain of the geysers were only in 

 eruption when the wind blew from a particular quarter. 



lY. — On an Accueate and Eapid Method of Estimating the Silica 

 IN AN Igneous Eoce:. By J. H. Player, F.Gr.S., F.C.S. 



THIS paper describes a method of estimating the silica in igneous 

 rocks by 



1. Fusing the finely ground rock with a flux prepared by mixing 

 carbonates of potash and soda and nitrate of potash. 



2. Disintegrating the glass so obtained by the action of strong nitric 

 acid. 



3. Driving oif nitric acid at a temperature just below 250°, thus 

 rendering all silica insoluble. 



4. Treating with hydrochloric acid, to leave the silica with some 

 impurity, for weighing after calcination. 



5. Separating the impurity by means of ammonium fluoride and 

 weighing it. 



Obseevations sue les Gteoupes sedimentaires les plus anciens du 

 NOED-ouEST DE LA Feance. Par M. Hebeet. (Extrait des Comptes 

 rendus des Seances de I'Academie des Sciences, tom. ciii., Seance 

 du 26 Juillet, 1886.) 



I IT this memoir, Professor Hebert submits to the Academy a view of 

 the Geology of North-Western France which differs in some points 

 from previous interpretations. Brittany and Western ITormandy present 

 difficulties such as have perplexed British geologists in many of the 

 tracts occupied by the older rocks. French writers are agreed that in 

 North-Western France there are (omitting the true crystalline schists) 

 (1) a granite, (2) a great formation of phyllite, and (3) a series of 

 purple conglomerates and (?) shales [schistes). The last-named group 

 is overlain by the Gres Armoricain, and over this sandstone come shales 

 and slates, with Acidaspis Buchii, and more than one species of Placo- 

 paria. These Placoparia beds, with the same Acidaspis, occur in our 

 Salopian area, not far above the grits of the Stiper Stones ; so that the 

 Gres Armoricain may be safely regarded as Arenig. But below this 

 horizon, the interpreters of Brittany geology find the materials for 

 widely diverging opinions. The theory of M. Hebert may be thus 

 summarized. 



The oldest of the three rock-masses is the granite. It furnishes 

 numerous rolled blocks to a conglomerate in the phyllite series, and it 

 never penetrates the adjoining strata in veins. At its junction with the 

 phyllite, the granite is in the state of sand, and the phyllite also is 



