144 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [NoV. 29, 



south of Oporto near Grijo and Yilla da Feira, some peaks of sienite 

 which interfere with the regularity of the series ; but notwithstanding 

 these and other irregularities, there is an approach to parallelism in 

 the position of the various members of this system of crystalline 

 rocks, the granite of Oporto appearing to be the lowest member, and 

 to be overlaid on both sides by micaceous schists, gneiss, and micaceous 

 or chloritic schists. The district to which the above remarks apply 

 is about twenty miles wide, measuring it from S.W. to N.E., at right 

 angles to the strike of the rocks. 



The foliation both of the schists and the gneiss usually strikes 

 about N.N.W., that is, parallel to the principal axis ; but it dips in 

 contrary directions on the two sides of the axis, being usually to the 

 S.W. on the S.W. side of the Oporto granite and to the N.E. on the 

 N.E. side of it ; therefore in each case the foliation dips away from 

 the granite as if in the form of an arch or saddle. On the western 

 side of Oporto, the space occupied by micaceous schists and gneiss, be- 

 tween the granite and the iVtlantic north of the Douro, is about two 

 miles wide, so that only a small portion of the western side of the 

 arch of the cleavage-planes is here seen ; but on the eastern side of 

 the granite the foliation and cleavage dip to the N.E. for a width of 

 about ten miles, ending with, a line of perpendicular cleavage in the 

 middle of the clay-slates about two miles east of Vallongo : thus the 

 cleavage and foliation appear to form an irregular arch over the band 

 of granite, of which the diameter if fully seen would be about twenty- 

 five miles. Beyond the line of perpendicular cleavage, the cleavage- 

 planes again dip S.W. and belong to another elevation. These phse- 

 nomena coincide in principle with those which I have described in 

 Wales, Cumberland, and Devonshire ; and help to confirm the rule 

 that the planes of cleavage are arranged in a sort of arch over the 

 axis of elevation of the rocks of the district. 



On both sides of the granitic axis the angles of dip of the foliation 

 vary from 60° to 80° with much irregularity. In the gneiss the folia- 

 tion is frequently wavy or even violently curved ; but viewed on a 

 large scale, its direction is parallel to that of the foliation of the 

 schists. 



Most of the quartz veins in the micaceous schists are parallel to 

 the foliation of the schists ; and where the mineral character of the 

 schists varies, the line of change is usually parallel to their foliation. 



Where the sedimentary slates rest on the crystalline schists at the 

 edges of the district of igneous rocks, the cleavage of the slates is 

 parallel to the foliation of the schists. 



Thus we find a general approach to parallelism in the foliation of 

 the crystalline rocks, in the direction of the majority of the quartz veins, 

 in the line of change in the minor mineral characters of the schists, 

 and in the cleavage of the sedimentary slates, the whole of which cor- 

 respond in strike with the line of the granitic axis of elevation of the 

 district. A similar parallelism has been pointed out by Mr. Darwin 

 in various parts of South America, between the planes of foliation of 

 the gneiss and mica schists and the planes of cleavage of the slates * . 

 * Darwin, Geological Observations oa South America, chap. 6. 



