ANNIVERSARY ADDRESS OP THE PRESIDENT. IxXXl 



sometimes contains masses and veins of granite, in other places it is 

 interstratified with calcareous bands or nodules, metamorphic, and 

 so charged with magnesia as to be useless for burning. It is gene- 

 rally stratified, and rises into more rugged crests and crags than the 

 granite, though this is not so much the case in the province of 

 Madrid itself as in the neighbouring districts. 



With regard to the origin of the gneissic rocks which were formed 

 by sedimentary action, the author observes that they are not all of 

 the same age. In the province of Madrid the gneiss is decidedly 

 metamorphic and contains beds of limestone which do not occur 

 in the Silurian formation. The gneiss has undergone much less 

 decomposition than the granite, although it appears to have been 

 eroded on a very large scale. This strikes Don Casiano as w^orthy 

 of notice, inasmuch as the granite contains 30 per cent, of quartz, 

 whereas in the gneiss this latter substance is almost entirely 

 wanting, and of the three component elements of both these rocks 

 the quartz is the least subject to decomposition. Other remarkable 

 instances of the decomposition of the gneiss are then described and 

 commented on ; but my space and your time will not allow me to 

 enter further into their consideration. 



The author then describes the Silurian formation, which, however, 

 is not of any great extent in this province ; it consists chiefly of 

 slates, sandstone, and quartzite, the former being most preponderat- 

 ing. E/Oofing-slate does not occur here, though it is found in the 

 neighbouring provinces of Segovia and Guadalajara. The amount of 

 denudation in former periods has been very great, and the quartzites 

 which alternate with the slates having oifered greater resistance to 

 this action are now seen standing out and forming rough and rugged 

 crests. After describing the principal strikes of these beds, which 

 oscillate from N. 20° E. to IS". 22° W., and the metamorphic action to 

 which they have been exposed, the author proceeds to describe the 

 fossils which have been found in them. 



These are very scarce in the province of Madrid, though many 

 casts of Graptolites have been found in that of Segovia. Those de- 

 scribed here are : — Cruziana six species ; Scolithiis, OrtJioceras, Lin- 

 c/ula, one each. Several forms of Acephala have also been met with, 

 but in too imperfect a condition to allow of their being satisfactorily^ 

 determined. These are followed by an account of the veins and 

 minerals which occur in the granite, gneiss, and Silurian beds ; these 

 are both metalliferous and non-metaUiferons. The latter consist 

 chiefly of pegmatite, quartz, and diorite : the metalliferous veins con- 

 tain a little silver, galena, sulphate of barytes, fluorspar, and iron 

 pyrites. These metals occur sometimes in detached masses or no- 

 dules, and sometimes disseminated throughout the whole substance 

 of the rock. Between the years 1841 and 1860 there was a great 

 development of mining enterprise, but the results were anything 

 but satisfactory. 



The author then describes in detail the various minerals found in 

 the granitic, gneissic, and Silurian rocks, and concludes this portion 

 of his work with an account of the different systems of elevation 



vol. XXII. / 



